A revert is to undo all changes made to an article page after a specific time in the past.  The result will be that the page becomes identical in content to the page saved at that time. However, in the context of the English Wikipedia three revert rule, a revert is defined far more broadly as any change to an article that partially or completely goes back to any older version of an article.
A partial revert is accomplished either by an ordinary edit of the current version, or by editing an old version. The former is convenient, for example, for a partial reversion of a recent addition, while the latter is convenient for a partial reversion of a deletion.
Below is some Wikipedia-oriented text on reversion.
Reverting
Deceleration
Stacking 
Extraction 
Blocking 
Stretching
Flexion
Swelling 