Aggressive (often abbreviated as aggro) is a term used to describe something (typically a mob) that will attack without provocation.
Aggressive mobs will initiate combat with the player. This is as opposed to a passive mob which will only attack if attacked first. Mobs can have different levels of aggression, which determine how quickly (or if) they will attack.
Following are the types of aggro mobs from least aggressive to aggressive on sight.
Mobs of this kind will attack if you lower your guard by sleeping. Resting will not trigger them to attack, however.
Some mobs will remember that you fought with them and will attack you if they meet you again. This is often indicated with the phrase "X recognizes you as murderer!" Other mobs are able to spread the word that you attacked them, which can cause an entire city of mobs to recognize and attack you in the worst of cases.
These mobs will attack only players who are of a specific alignment, for example an evil monster might attack only neutral or good characters.
Consider <mob> will sometimes tell you if a mob is aggro or not with the message "X would not hesitate to come after you."
An area that contains a high number of aggressive mobs. As a result, these areas are generally harder but tend to grant more experience.