From Liquipedia StarCraft 2 Wiki
Fast Expand is also referred to as FE. It is used to describe when a player of any race takes their Natural Expansion fairly quickly.
Generally these are the timings for a build to be referred as a fast expand:
- Zerg - Taking your expansion before Spawning Pool finishes.
- Protoss - Taking your expansion with one or no Gateways.
- Terran - Taking your expansion with only one Barracks.