To me, the whole thing is a "bunch"; a single stalk as in (2) is a "stick". I don't know that I would refer to the leaves by any other term, but I suppose it could be the "head".
Edit: A web search shows there is considerable disagreement between terms used by botanists ("stalk" for the whole thing and "rib" or "branch" for one part of it), Americans ("stalk" for what botanists call a "rib") and Britons ("stick" for a "rib", minus the leaves, and usually "bunch" for a botanist's "stalk"). I might not have got all the terms/differences quite correct.
I would call 1. a head or bunch and 2, a stick of celery, a stem of celery or a celery stalk.
I see on google that some refer to 1. as a rib and others to 2. as a rib but I've never used "rib" for either.
Usually I see "celery sticks" meaning "small pieces, cut up and prepared for eating". Appetizers often include "carrot sticks" and "celery sticks" that you can hold in your fingers and dip into one of the dipping sauces, then eat. (See picture) Grocery stores sell "carrot sticks" and "celery sticks" already cut up into short slices to eat.
If I am talking about the whole plant, I would call it a "bunch". Each individual part of the bunch is one "stalk". (See picture)