What part of speech is critics?

"Critics" can be either a noun or an adjective, depending on how it's used in a sentence.

Noun:

* "The critics gave the movie a thumbs down." (Here, "critics" refers to people who write reviews.)

Adjective:

* "The film received critics' praise." (Here, "critics'" is possessive and describes "praise".)

Therefore, you need to consider the context to determine the part of speech.