Is the word meeting an adjective?

No, the Word 'meeting' can be used as either a verb or a noun. Meeting can mean a place where you gather, i.e.: Sally went to a meeting. It can also be used as a verb, to mean that you will meet someone, i.e.: Xavier is meeting his cousin this afternoon.

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"Meeting" can be used as an adjective, although not very often.

In "a meeting place," or "a meeting hall," the Word "meeting" modifies the nouns "place" and "hall." (Some, however, would claim that "meeting place" and "meeting hall" are simply compound nouns.)