Two meanings related to each other:

1. The lowest gear ratio on a multi-speed derailleur bicycle; smallest chainring in front and the largest at the back.

2. The smallest chainring on a crank with triple chainrings.

This term describes the lowest gear ratio possible, which means the bike is on the smallest chainring in the front and the largest chainring in the back. On flat roads, a granny gear feels like effortless spinning. On steep climbs, sometimes the granny gear is necessary for survival.