The big bang theory is defined as the theory that the universe may have been created in a huge explosion at least 12 billion years ago, or is a successful comedy series created by Chuck Lorre in 2007 about four geeky scientists and a waitress neighbor. (noun)
An example of the big bang theory is the accepted scientific theory that the universe started when a hot dense state expanded and exploded causing the universe to cool and start to convert energy into its current state.
See big-bang theory in Webster's New World College Dictionary
See big-bang theory in American Heritage Dictionary 4
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