Understudy

1. n. One who studies another’s part with a view to assuming it in an emergency.
2. v. t. & i. To study, as another actor’s part, in order to be his substitute in an emergency; to study another actor’s part.

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.