What is the word "which", a pronoun or an adjective?

Question

In this sentence I'm not sure if the word which is a pronoun or an adjective:

The helicopter, which spirals down from the clouds.

Answer 1

Well, in this sentence fragment, "which" is a relative pronoun whose antecedent is "The helicopter."   It begins the  adjective clause "which spirals down from the clouds."  You need to finish it though; the sentence doesn't have a verb.  "Spirals" belongs to the subordinate clause.  I don't know what you had in mind, so I'm just suggesting "The helicopter, which spirals down from the clouds, is blue.  

You can make it a complete sentence by deleting the "and."  This would make "lands" the main verb.  If you don't do this, it still isn't a complete sentence.

Answer 2

Your example isn't a complete sentence.  However, your question can still be answered.

The words "which spirals down from the clouds" most likely constitute a nonrestrictive relative clause.  Without more context, I can't be completely sure whether it's a restrictive or nonrestrictive relative clause, although the comma and the word "which" both make the nonrestrictive case more likely.

The entire clause acts as an adjective modifying the word "helicopter".  The word "which", as the subject of the clause, is a relative pronoun.