Aggressive

"Aggressive" has two meanings in Runescape

A style of combat
In the aggressive style, experience is given to strength and the combat style offers a higher maximum hit, but risks more misses (zeros) and being hit more often.

An aggressive monster
Any monster or NPC that is defined as aggressive, will attack you without any provocation other than walking near enough for it to consider you a target.

Non-Wilderness
Aggressive monsters seem to ignore you once you are over twice their combat level, while some seem to be a lot more persistent in seeking you out, though most are stopped by any obstacle which requires then to move away from you to get round it.

Giant rats (level 3) will bother low level characters, including level 6.

Other major aggressive monsters are:
 * Dark Wizards, their magic attack is especially dangerous against armour
 * Draynor Jail Guards (but not city guards)
 * Scorpions and King Scorpions - the level 32 King Scorpions in the Dwarven mine are probably the worst mine monster outside the wilderness, but level 65 combat should provide immunity.
 * All dungeon monsters, including those in dungeon mines.

Wilderness
All monsters in the wilderness are aggressive, even if you can swat them like flies, This causes annoyance to high level magic users, as they can waste good combat runes swatting unworthy opponents in a single blast.