| v. i. | 1. | to move in a reverse direction; - used of vehicles or animals. |
| 2. | to accumulate due to a blockage of flow; | |
| v. t. | 1. | to serve as a backup{3} for (another person or persons); |
| 2. | (Computers) to make a backup{5} of; |
| Verb | 1. | back up - give moral or psychological support, aid, or courage to; "She supported him during the illness"; "Her children always backed her up" Synonyms: support |
| 2. | back up - move backwards from a certain position; "The bully had to back down" | |
| 3. | back up - establish as valid or genuine; "Can you back up your claims?" Synonyms: back | |
| 4. | back up - make a copy of (a computer file) especially for storage in another place as a security cop; "You'd better back up these files!" | |
| 5. | back up - become or cause to become obstructed; "The leaves clog our drains in the Fall"; "The water pipe is backed up" |