In terms of television, the term "prime time" designates television audience patterns applicable to the U.S. television viewership. The exact hours considered prime time vary with the different U.S. time zones.
Maximization of viewing levels based on the two coastal time zones, Eastern and Pacific, determines prime-time television hours. High viewership correlates with more valuable advertising time.
Prime-time hours as of January 2010 break down to 6 to 10 pm in the inland (Central and Mountain) time zones and 7 to 11 p.m. in the coastal (Eastern and Pacific) time zones, according to the article "Prime Time" by the Museum of Broadcast Communications (Museum.tv).
Basically, prime-time television programs are any programs broadcast during prime-time viewing hours. Historically, the television industry has demonstrated particular patterns of prime-time programming over intervals of time.
In early days of television, variety shows and westerns enjoyed popularity. As of 1992, according to Television Network Prime-Time Programming, 1948--1988, trends included continued appeal of drama and ongoing growth of the situation comedy. As of January 1, 2010, an observable trend is the rise and dominance of the reality show.