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Definition: a command that allows the computer user to reverse the effects of the most recent operation. If “Undo” is dimmed (printed in gray), it is not possible for your program to undo a command (perhaps you have performed an intermediate step—or perhaps the most recent command was a complex operation that cannot be undone). For this reason, it is wise to save different versions of the file as you work so that you will have a recent version to revert to in case of disaster.
Source: Downing, Douglas, et al. Dictionary of Computer and Internet Terms. 10th ed. New York, 2009. Print.