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1.
An overbearing entity in a specific industry or sphere of activity. A seemingly unbeatable presence always to be reckoned with; whose experience, influence, and skill threatens to defeat competitors with little effort.
In the software industry, Microsoft would be considered the 800-pound gorilla.
The New York Yankees or Manchester United might be considered the 800-pound gorillas of their respective sports.

by K June 18, 2006
 
2.
a very strong and/or menacing entity or issue that is obvious but not dealt with
Abuse suffered in prison is an 800-pound gorilla that is undermining justice.
by The Return of Light Joker April 13, 2009
 
3.
The big, dominant factor regarding some topic.
“I can’t think of anybody who had more impact than Dr. Dobson,” on rousing evangelicals to the polls, Richard Viguerie, a GOP direct-mail guru, recently told U.S. News & World Report. “He was the 800-pound gorilla.”
by The Wu Factor November 15, 2005