It means he's still stored on your game's memory even though he moved out some time ago..
I don't remember how many villagers you have to let move out before they'll appear on your Main Street.. However, once 16 villagers have moved out from your town after that one moved out.. They'll stop appearing there (them stopping appearing also means you know when your 16 villager cycle is complete.. As once a villager has moved out, you need 16 more to move out before you can get them back into your town)