A “mega-donor” (or"megadonor”) is someone who gives a large sum of money to a person or a cause. Political campaigns actively seek “megadonors,” but no specific dollar value is attached to the name.
 
“Mega-donor” was cited in print in 1991 (regarding a donation to thearts) and in 1992 (regarding $100,000 donations to the Republication National Committee).
 
 
19 June 1991, The Orange County Register (Santa Ana, CA), “People Watch” by Elizabeth Magers, pg. E6, col. 4:
Chorale mega-donor Georgia Frontiere, attending with Earl Weatherwax, praised the concert.
 
26 July 1992,The New Mexican (Santa Fe, NM),  “Soft money: once a loophole, now an institution” by Charles R. Babcock (Washington Post), pg. A-4, col. 4:
The RNC calls its mega-donors “Team 100”—a group that now numbers about 275 individuals who have given at least $100,000 of personal or corporate funds to the committee.
 
18 August 1992, The Orange County Register (Santa Ana, CA), “People Watch” by Carol Humphreys, pg. E1, col. 2:
The mega-donors to the new Bowers Museum of Cultural Art were stunned.
   
13 May 1994, Marietta (GA) Daily Journal, “House spares Appalachian Regional Commission” (AP), pg. 3B, col. 3:
“Florida is a mega-donor state. ... We want to level the playing field,” he (Rep. Porter Goss, R-Fla.—ed.) said.
 
17 December 1995, Rockford (IL) Register Star, “To early to tell Rose Bowl’s effect on NU” by Mike Lopresti (Gannett News Service), pg. 7G, col. 4:
Since 1987, 17 mega-donors have contributed at least $5 million each.
 
21 March 2002, Daily Herald (Arlington Heights, IL), “Analysts warn bill only redirects cash: Campaign reform ayt not halt special interest funds” (Reuters), sec. 1, pg. 6, col. 2:
“You’re reducing the power of the mega-donation but doubling the power of the mega-donor,” said Larry Makinson, a senior fellow at the Center for Responsive Politics.
 
Washington (DC) Examiner
Megadonor: Tea Party ‘ruined’ GOP
By JOSEPH LAWLER • 11/4/14 1:19 PM
The Tea Party has “completely captivated and ruined” the Republican Party, and will doom the party’s chances of reclaiming the presidency unless it is checked, says Jon Huntsman, Sr.
   
Business Insider
Major GOP megadonor Sheldon Adelson just got some of his money back from Marco Rubio
HUNTER WALKER
Jul. 23, 2015, 5:53 PM
Billionaire casino mogul Sheldon Adelson, one of the Republican Party’s biggest megadonors, got some of his money back from the presidential campaign of Sen. Marco Rubio of Florida last month.