Definition of pretty adverb from the Oxford Advanced Learner's Dictionary

Oxford3000

pretty

adverb
ˈprɪti
 
; ˈprɪti
 
 
(with adjectives and adverbs) (rather informal)1 to some extent; fairlyI'm pretty sure I'll be going.The game was pretty good.It's pretty hard to explain.I'm going to have to find a new apartment pretty soon.2 veryThat performance was pretty impressive.Things are looking pretty good!
Idioms

pretty much/well

(British English also pretty nearly) (North American English also pretty near) (informal) almost; almost completelyOne dog looks pretty much like another to me.He goes out pretty well every night.The first stage is pretty near finished.
more at be sitting pretty at sitUsage noteUsage note: quite / fairly / rather / prettyLook at these examples:The exam was fairly difficult.The exam was quite difficult.The exam was rather difficult.Quite is a little stronger than fairly, and rather is a little stronger than quite. Rather is not very common in North American English; pretty has the same meaning and this is used in informal British English too:The exam was pretty difficult.In British Englishquite has two meanings:I feel quite tired today (= fairly tired). With adjectives that describe an extreme state (‘non-gradable’ adjectives) it means ‘completely’ or ‘absolutely’:I feel quite exhausted. With some adjectives, both meanings are possible. The speaker’s stress and intonation will show you which is meant:Your essay is quite good (= fairly good — it could be better); Your essay is quite good (= very good, especially when this is unexpected).In North American Englishquite usually means something like ‘very’, not ‘fairly’ or ‘rather’. Pretty is used instead for this sense.