Wikipedia:Five percent rule

Several RFCs have established that third-party candidates must poll over five percent to be included in an infobox. The only exception is if only one candidate polls over five percent, in which case the second-place finisher may be included if determined appropriate by local consensus. This does not strictly apply to parliamentary elections, where other criteria may be used to include candidates if deemed appropriate. In United States presidential elections, candidates must also have enough ballot access to win.

Origin
The notion of a "five percent rule" appears to originate with Walter Dean Burnham's 1970 essay Critical Elections and the Mainsprings of American Politics. Page 4 of Three's a Crowd: The Dynamic of Third Parties, Ross Perot, and Republican Resurgence states "most scholars follow Walter Dean Burnham, who defined 'successful' third parties as those that attract at least 5 percent of the vote. (Burnham 1970)".