What category does the word Nearly fit into?

"Nearly" fits into several categories, depending on how you want to classify it:

* Part of Speech: Adverb

* Function: Modifier (It modifies the meaning of a verb, adjective, or another adverb)

* Semantic Category: Approximation or Degree

* Grammatical Category: Almost/Not Quite

* Lexical Category: Adverb of Degree

Ultimately, the most appropriate category depends on the specific context in which you're using the word.