there is a sentence below.
- I don't watch a lot of TV anymore.
In this sentence, what is the definition of "of"?
there are lots of definition concerning of "of" in dictionary.
but I don't know what is excatly.
It is a preposition, and is used to express the idea of "a part"
We say
four of the cats
Meaning not all the cats, only four. Or
Most of the cats
or even
All of the cats
Meaning the part is actually the 100%
So in your example
A lot of TV
means "not all TV programs, but many TV programs"
Other languages express partitives in other ways, and may not use a partitive at all in this construction. You will probably find it easier to understand if you think "A lot of" as being a single idiomatic phrase, and not try to understand the meanings of the the separate words.
The phrase "A lot of" means "many" or "much"