Definition of zillion noun from the Oxford Advanced Learner's Dictionary

 

zillion

 noun
noun
BrE BrE//ˈzɪljən//
 
; NAmE NAmE//ˈzɪljən//
 
(informal, especially North American English)
 
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a very large number There was a bunch of kids waiting and zillions of reporters. This movie is a zillion times better than her last one. Word Origin 1940s: from Z (perhaps as a symbol of an unknown quantity) + million.
See the Oxford Advanced American Dictionary entry: zillion