A named marker tied to an entity, usually reflecting its narrative state, that add stats or applies specific effects.
For heroes, aspects are just facts about the hero. Stuff like "human", "female", "attractedToMen", "hunter", "missingLeftArm". Aspects can carry stat modifiers and Effects. Aspects are the workhorse of the dynamic content in the game, almost everything you need to know about a hero, site, anything really, can be accessed as an aspect (other than stat values).
A special subcategory of aspects is hooks.