1. the period in European history from about a.d. 476 to about 1000.
2. the whole of the Middle Ages, from about a.d. 476 to the Renaissance.
3. (often l.c.) a period or stage marked by repressiveness, a lack of enlightenment or advanced knowledge, etc.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.