prime time

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The time between 8 and 11 PM or between 7 and 10 PM, depending on the time zone, in the United States and Canada, especially with respect to programming on television and cable networks. On Sunday, prime time begins an hour earlier.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun obsolete Spring.
• noun obsolete A new period or time of youthfulness; the beginning of something.
• noun television, radio The block of programming on television during the middle of the evening, usually between 19:00 and 23:00
• adjective television, radio Showing during prime time.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the hours between 7 and 11 p.m. when the largest tv audience is available