A revenant is a visible ghost or animated corpse that returns to terrorize the living, usually people it knew in life. Unlike the zombie, a revenant is not under another person's control, and has returned from the grave on its own for a specific and often personal purpose.
In 4.01 Lazarus Rising, Bobby accuses Dean of being a shapeshifter or a revenant. Dean proves he's neither by cutting himself with a silver knife.
In 8.06 Southern Comfort Garth takes a call from a hunter asking how to deal with a revenant. Garth tells him to get a casket and some silver spikes, then nail it in and bury it. He also warns that getting bit will hurt like hell, but will not turn you into one.