What defines a "roll"?

"Roll" is an extremely broad culinary term without a single canonical definition. You can very likely call your preparation a "chicken roll" and nobody would object.

In American cuisine, the term "roll" used on its own is often short for "dinner roll", a type of leavened single-serving bread:

Used in combination with another food ("[x] roll") the term usually denotes some form of sandwich, although confusingly few of these use a dinner roll as the bread. Here's the "lobster roll" that you reference, which usually uses a hot dog bun or similar:

Similar terms often denote sandwich-like foods where the bread is "rolled" around the fillings, like this "chicken roll":

There's also the "pepperoni roll", which takes this even further into a tightly rolled spiral which is then baked:

The term "roll-up" is also sometimes used for this kind of spiral roll, especially when the result is sliced and served as an appetizer or finger food:

New Jersey also has a local specialty called a "pork roll", which isn't rolled at all:

So as you can see, "roll" can be used to describe a wide variety of foods that involve bread, and sometimes fillings, which are sometimes rolled, and sometimes not. If you want to call your creation a roll, there's nothing stopping you.