The definition of back up means someone or something that is a substitute or reinforcement.(noun)
An example of back up is a second team of Navy Seals waiting to raid a compound if the primary team needs assistance.
Back up is defined as to walk, drive or move in reverse.(verb)
An example of back up is for a truck to move in reverse to line up with a loading bay.
Back up means to copy or save computer data on a location other than your main computer hard drive.(verb)
An example of back up is when you copy your pictures to a CD.
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