1. verb To move backwards. This phrase is commonly used to refer to maneuvering a car in reverse. Back up, you're standing too close to me! If you back up a little, then your car will be completely in that parking space.

2. verb To save copies of computer files in another place, in case the original files become inaccessible. If you didn't back up your files before the computer crashed, they may be lost forever.

3. verb To become clogged and impassable. I'm calling the plumber right now because the toilet is backed up again. Traffic is totally backed up, due to all that construction.

4. verb To return to an item previously mentioned. Whoa, back up—Janet and Jim are getting married?

5. verb To support or assist someone. In this usage, a noun or pronoun can be used between "back" and "up." You didn't see that meeting on the calendar, either? OK, please back me up on this, so the boss doesn't think I'm an idiot.

6. noun An alternate to be used if it becomes necessary for some reason, typically the failure, ineffectiveness, or the absence of the original. The phrase is typically written as one word. I brought an extra pen as a backup, in case this one runs out of ink during the exam. I hope Tony asks me to the prom, but, if not, I have my best friend Bill as a backup.

7. adjective Available for use as an alternate if it becomes necessary for some reason, typically the failure, ineffectiveness, or the absence of the original. The phrase is typically written as one word. I brought three backup pens, in case this one runs out of ink during the exam. I'm the backup quarterback, but I still have to know all the plays in case I get in the game. I hope Tony asks me to the prom, but, if not, I have my best friend Bill as a backup date.

8. adjective Appearing in the background in support of a main act or performer. The phrase is typically written as one word. She's auditioning backup dancers for her world tour today.