freewheel
C2Substantiv/US /frˈɛˈɛwhˈɛˈɛl/ freh-eh-wheh-EHL; UK /frˈɛˈɛwhˈɛˈɛl/ freh-eh-wheh-EHL/
A clutch fitted in the rear hub of a cycle, which engages the rear sprocket with the rear wheel when the pedals are rotated forwards, but permits the rear wheel to run on free from the rear sprocket when the pedals are stopped or rotated backwards.