A player who lines up in the runningback or fullback position, but whose primary job is to block a defensive player or open up a hole for the ball carrier.
Some backs are known purely for their blocking ability, meanwhile others like Jim Tayler and Paul Horning were known for both carrying the ball and blocking. Many of the best runningbacks in NFL history credit blocking backs for their success.
One of the best ever was Lorenzo Neal, from 2003-2007 he helped LaDainian Tomlinson put up incredible rushing numbers. He also blocked for Eddie George, Corey Dillon, and Warrick Dunn in their 1,000+ yard rushing seasons.