Alternator Clutch Pulley
This is a one way drive pulley which drives the pulley in the normal direction of travel and freewheels in reverse. If it doesn't do that, then early failure of the tensioner assembly and idler pulley will result. Do not use a solid pulley in its place or this will also happen. The failure of this pulley is the usual cause of a bad tensioner or idler pulley so if you need either one check this pulley at the same time as it may need to be replaced as well.