Aggressive, or Aggro refers to enemies in the game that will attack the player without any sort of provocation. Some enemies have one or multiple detection types, and provided said conditions are met will (usually) growl and go after the player. If an enemy checks as too weak to the player, aggressive monsters will not respond.
A cone extending about 12' from the front of your opponent will cause aggression. This can be avoided with Invisible unless the opponent has True Sight.
A radius of 8' from the enemy will cause aggression. This can be avoided with Sneak unless the opponent has True Sound.
If you are in a state of low HP (Yellow), enemies within a radius of 8' will cause aggression. As your HP drops farther, this radius increases.
If you are healing/meditating, enemies within a radius of 8' will cause aggression.
If you are casting a White, Black, Blue or summoning magic spell, enemies within a radius of 8' will cause aggression. This does not affect songs or ninjutsu.
Some monsters, such as Hpemdes, will aggro the player, but as long as they are not damaged will not attack the player. They can be attacked, have spells cast on them and they won't engage until you do damage. This can be useful for skilling up magics, such as musical skills, as they often have no damage associated with them.
When an enemy can link, if they see or hear (based on detection) a fully engaged engaged monster, they will join in on the attack.