Aggression is simply 'moving toward' something. Aggression, as a quality of a person, speaks to one's relationship to one's natural impulses. Emotions and desire engender impulses. Aggression is moving toward something on the energy of an impulse. In human affairs there is always external resistance, and so "aggression" is the ability to move toward a constructive goal against some resistance.
Aggression is not the egotistical intent to get more than others, get ahead of others, or get some ill-gotten gain. The word for that is greed. Aggression is the biological and interpersonal process by which impulses and desire are transformed into action.
Now aggression can be distorted, and one of these distortions can be violence. Violence, broadly defined, is imposing one's will on another. More narrowly violence is harming another. Most personal violence is about revenge and not about getting something. Episodic destructiveness is not aggression. In fact most destructive behavior comes from the suppression and holding of feeling and then its distorted eruption. Personal instrumental (goal-directed) violence is rare and implies lack of feeling. Violence is about control, and actually impulsive. The confusion of aggression with violence is a large part why aggression is distrusted. It is true that an incapacity for aggression may translate into a lowered capacity for violence, but this is like chopping off legs to prevent kicking.
Another distortion of aggression is passive-aggression. While passive aggression is a trait that has many descriptions, for this discussion it may suffice to say that the impulse to 'get to others' is entirely disguised but usually successful in a frustrating way. A common reaction is, "If life with such apparently well-intentioned efforts is so hard, who needs aggression?" But life is much easier when straightforward. The solution to passive-aggression is not to remove the aggression, but remove the passivity!
Aggression is not just persistence. Mere repetition may succeed in attaining a goal, but aggression seeks to understand and really engage with the resistance. Likewise, sheer frenetic, vigilant activity may succeed in achieving a goal, but it usually consists in providing or offering what is already accepted or wanted, and so also not quite a use of aggression. Neither is aggression use of the will. Will power is used to act contrary to feeling, aggression always follows feeling.
For this discussion, it is assumed that impulses are good and natural, and aggression is the energetic process that brings this goodness and naturalness into the world. Now, mature persons do not act immediately on all their impulses, but this is not because they feel their impulses are alien or bad. This is the result of self-possession. The basic impulse is held for a better time or circumstance, according to the reality principle. Aggression implies the capacity to hold feeling in awareness long enough to shape it into a creative response.
Many people feel stuck, because they believe one way, and act another. In this situation, beliefs are untested. Aggression presses to align beliefs and actions, which is harmonizing to the person, certainly, and almost always, to relationships as well. Of course beliefs may change in the process, but also actions will happen where before there was passivity.
Part of aggression can be taking a position in a conflict. This is different from taking a side in a conflict. A position is a belief in what should happen that one is able to back up with actions. It makes both unilateral actions and collaboration possible. (Of course, people really skilled in conflict resolution think of interests more than concrete positions, but in the sense meant here, interests are ranges of positions).
Part of aggression is also telling people, when appropriate, what to do (and of course people told still have the right not to do it if it seems wrong to them) Many 'captains of industry' and entrepreneurs have no real skills other than sufficient aggression to tell people what to do. For this they are usually well rewarded. Many people know what to do but cannot tell others to do it, and some people know what to do themselves but still need to be told to do it. Occasionally someone doesn't know what to do, and they may only need to be told what to do but they may also need to be told to do it. Parents are often reluctant to tell children what to do, and often confuse them with extensive moralizing about what is right in an effort to manipulate the children into doing what they want. There is a way to tell someone what to do without dis-respecting or dehunanizing them but this requires understanding that aggression and empathy are not incompatible.
To the extent a person is less able to really 'possess' an impulse, there is a tendency to handle it in three ways: 1) enact counter-impulses, that is to say a reaction formation, 2) be generally inhibited and over-controlled, or 3) act out substitute impulses either in a repetitive fashion (compulsive behavior) or in a chaotic fashion (impulsive behavior). The more the underlying impulses are distorted in these ways of course, the more they leak out someway into interpersonal affairs, and the more justified it seems to stifle them. This is a 'vicious circle.'
The first two categories above, reaction-formations and over-controlled-ness are encouraged in our culture because they do seem to limit maiming and killing (at least directly, though many 'architects' of mass destruction, are in fact over-controlled). Compulsiveness and impulsiveness may be frowned upon, but actually more tolerated than aggression.
Mere drive to obtain an idea or goal regardless of the consequences or process is psychopathy, with which aggression is also frequently confused.
What matters is not the strength of the idea to get something, but rather the energy level and energy structure to support movement toward the goal, and the ego skills to negotiate interpersonal complications along the way. Just as there are driving skills that never get developed if one never takes the car onto the highway, ego development is limited if aggression is limited.
Aggression is a key element by which character can be evaluated. For instance, the schizoid or creator character can formulate constructive goals and even mentally anticipate obstacles but tends to leave the goal as abstract 'for later.' There is a complete disconnect between a goal and the impulse to go after it. The oral or communicator character also can formulate goals and sets off impetuously but tends to quickly feel that obstacles are too great and retreats. They may call for someone else to intervene or apply a rule. The swollen or includer character may go about a goal even more impetuously but tends to get distracted and also retreats if resistance is great. The psychopathic or inspirer character also acts quickly and quickly loses sight of the original goal and gets lost in the experience of being influential or dominant. The masochistic or consolidator character clearly experiences impulses but holds in powerfully.
Where aggression is low, there is a tendency not to act on one's judgment, but seek seek certainty. This can contribute to inaction, but it is also a trait that contributes to conscientiousness. On the other hand, those with greater aggression tend to trust their own judgment and act on it more readily. This aids decisiveness. There is some antagonism between conscientiousness and decisiveness. We live in an economy where there is room for only a few decisive people, but there is room for many, many conscientious people. This possibly contributes to a societal disapproval of aggression (both as it is misunderstood and as it is meant here.)
Also where aggression is low, the person is at risk for bitterness. This is because those who are aggressive and thriving are perceived as cheating or acting as bullies by pushing others aside. A world view of unfairness develops
The rigid or achiever character is best able to apply aggression but tends to organize all activity around aggression, including personal relationships. In general, women tend to be more aggressive (try to bring about desirable conditions) in the family or close relationships, men tend to be aggressive (try to make something happen) in the world.
To better understand the role of aggression, it may be helpful to contrast two other human traits: receptivity and passivity. Receptivity is the complement to aggression. To receive is to participate in another's impulse. If the impulse is directed at oneself then receptivity is taking it in. If the aggression is directed elsewhere, than receptivity is following actively and with conviction. Passivity on the other hand is not participating. Passivity results in neither aggression nor receptivity. Passivity may result in following but without conviction. There is a lack of true willingness.
The path of least resistance describes a trend that can develop where aggression is low. Excessive doing of unopposed things can be a compensation for difficulty doing what one really wants, that is busy-ness or frenetic activity can be a sign of low aggression. Drifting is moving from endeavor to endeavor based on interest and agreeableness, but moving away after reaching the point where some aggression is necessary (which it always will be in anything worth doing). Drifting looks like exploration for a while, but after a time the avoidance is more clearly seen. Drifting avoids developing the assurance, strength, and functional roots that working through provides. Some situations are toxic, in that aggression has no chance of helping and is in fact punished. Toxic situations are rare though, and have to be distinguished from situations that are merely difficult, competitive, impersonal, slightly biased, or that require new abilities.