In the Miniatures Handbook, aspects are something like avatars, but they are much more minor manifestations of power by deities or other powerful beings.
More broadly, an aspect is a particular name and shape taken by a deity before one world or group of worshippers. It is possible for a deity to die on one world, but remain alive on others, because only one aspect of the divinity's many facets has been destroyed.
To give another example, the goddess Nula may be worshipped under her aspect as Goddess of Fish or under one of her many other aspects, such as the Goddess of Cats. The Goddess of Fish and the Goddess of Cats may be treated as entirely seperate deities, but they are in truth one and the same.