Most of the time we say 單字 Dan Zi. (This is by far the most common way that me and my friends and everyone else would say) But actually it doesn't matter, 單字、字彙、辭彙、詞彙 would be understood by the native Chinese speakers :D
(Just sharing my own opinion, it might not be 100% professional but it's a good way of communicating with people.^^ )