Question

what does it mean for terrance to be \in his element\ at the convention?
terrance feels because he is in a huge crowd of people with the same as him.

Explanation:

The phrase "in his element" means being in a situation where one is most comfortable and proficient. In this context, since it's a convention and there's a crowd of people with the same [something], "hobby" makes sense as conventions often gather people with shared hobbies.

Answer:

hobby