Chainline is the angle of a bicycle chain relative to the centerline of the bike's frame. A bicycle is said to have perfect chainline if the chain is parallel to the frame, which means that the rear sprocket is directly behind the front sprocket. Chainline also refers to the distance between a sprocket and the centerline of the bike.
Bicycles with straighter chainline are more efficient due to the frictional losses incurred by running the chain straight around both sprockets but at an angle between them. This is the main reason that a single-speed bicycle is more efficient than a derailleur geared bicycle which is shifted into an identical gear combination. Single-speed bicycles must have the straightest possible chainline in order to prevent the chain from falling off. This is particularly important for fixed-gear bicycles, because chain derailment can cause such a bike's rear wheel to lock.