Aggression is a both a political and social concept. In politics and military affairs, it refers to taking the first move in a war - especially a war of conquest. The United Nations has proposed various treaties banning "aggression" but without giving any sort of definition of it at all.
In human society aggression is often synonymous with taking the initiative in any endeavor, such as sales, R&D or sports leadership.
Among animals of the same species, aggression is generally found among males competing for mates or for small group leadership.