Definition of all-time adjective from the Oxford Advanced Learner's Dictionary

 

all-time

 adjective
adjective
BrE
 
; NAmE
 
[only before noun]
 
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(used when you are comparing things or saying how good or bad something is) of any time one of the all-time great players my all-time favourite song Unemployment reached an all-time record of 3 million. Profits are at an all-time high/low.
See the Oxford Advanced American Dictionary entry: all-time