Aggro is a jargon word in MMORPGs, probably originally derived from the English words "aggravation" or "aggression", and used since at least the 1960s in British slang. In MMORPGs, such as STO, aggro denotes the aggressive interests of a monster/NPC.
Some examples are "We've got aggro!" and "Go aggro that Klingon".
For example, you may read of a player who died while fighting one mob because she aggroed another mob — that is, another mob decided to attack her because she got too close to it, or for other reasons. Aggro is closely associated with the concept of threat. If a mob is under attack by more than one player, then the mob will aggro onto (i.e., will begin to attack) the attacker with the highest threat.
Aggro is understood to be the condition of a particular mob attacking a particular character, while threat is the numeric value that each attacker has towards each character on its threat list. The basic behavior of aggro is controlled by the rules outlined below. Some mobs will have secondary attacks which have different targeting procedures; even for a normal creature, the target who has aggro is not necessarily the player highest on its threat list. Managing aggro is one of the most important aspects of grouping because it determines how much damage the group receives and where the damage ends up.
Holding aggro is done by (usually) one person (the tank) who uses his abilities to make the enemy attack him and no one else. Aggro control is the art and work which is done by all members of a party to force the mob to attack the tank(s).