from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A literary, dramatic, or cinematic work whose narrative takes place before that of a preexisting work or a sequel.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. In a series of works, an installment that is set chronologically before its predecessor, especially the original narrative or (perhaps improper usage) any narrative work with at least one sequel.
pre- + (se)quel.(American Heritage® Dictionary of the English Language, Fourth Edition)
Patterned after sequel using pre- ("before"). (Wiktionary)