What part of speech is the word tell?

It can be a verb, as is "tell me a story"

It can also be a noun. Normally, used as a noun, it means an attribute that unintentionally gives away information. Commonly used in Poker and other card games. For instance, "I knew he was bluffing. His tell is that he talks louder when he's bluffing." I don't know if that is an official definition found in the dictionary though.