1. Literally; n., a pin or keeper, placed through or around an axle, that prevents a wheel from slipping off of its axle.
2. Slang; n., Any part or person that is so vital to something that if it went missing, the whole thing would fall apart.
1. You have to pull the linchpin to adjust the bearings.
2. Someone has to take the snap, so the quarterback is the football offensive team's linchpin.