Would you say 当动词 or 是动词?
当 here means act as; be regarded as; be considered as.

是 is simply to be. 

Both words are common and can be used colloquially. In formal/academic context, I'd replace 当 with 作 or 当做.

If the word is not verb but can be used as a verb, you should say 当, e.g. 


  在古汉语里，很多形容词都可以当动词。


If the word is a verb, you should say 是, e.g. 


  服务这个词既是名词，又是动词。