A "revival" is traditionally any new production of a previously produced script. However, in recent years these have fallen into a variety of categories, especially when referring to musicals.
There have been Broadway revivals of My Fair Lady Hello Dolly, A Chorus Line, The King and I, Gypsy and others that have essentially recreated the original Broadway production (sets, costumes, choreography and, in some cases, casting).
Occassionally, as noted in the thread, an older vehicle will be revised due to dated or other-wise out of fashion material - sometimes substantially. Flower Drum Song comes to mind - the book was reworked or perhaps Crazy for You - which became a "new" musical, but was really a major revision of Girl Crazy.
There are also completely new, often highly illuminating productions of classic texts: Guys and Dolls, Most Happy Fella, Carousel and Oklahoma.
There is also the hybrid revue such as Fosse or Jerome Robbins' Broadway. These productions revive individual sequences from previously produced material. In the case of Fosse, these were simply recreations of Fosse's choreography and with the Robbins' piece, entire sequences were recreated including sets and costuming. Note that each of these were considered "new" musicals" during their runs.
As far as straight plays go, any production after the original staging is considered a revival, though it may be a regional or local premiere.
"Love Life. Live." Michael Bennett
