back·up / ˈbakˌəp/ • n. 1. help or support: no police backup could be expected. ∎ a person or thing that can be called on if necessary; a reserve: I've got a security force as backup [as adj.] a backup generator. 2. Comput. the procedure for making extra copies of data in case the original is lost or damaged: automatic online backup | [as adj.] a backup system. ∎ a copy of this type.3. an overflow caused by a stoppage, as in water or automobile traffic: there are long backups on all routes.