I mean technically, extremely reductively, yes…

… but ‘denim’ specifically refers to a way that cotton is done with a twill weave, that gives it more rigidity and durability and puncture/slash resistance…

… than what ‘cotton’ on its own, tends to refer to in the realm of fabrics, which is a much more flexible and breathable weave, that is also less rigid, durable, and damage resistant.

Before the advent and proliferstion of synthetic fibers… a whole lot of different fabrics… yes, are literally cotton, but they are wefted and woven in different patterns that give them both different visual appearances, as well as different physical properties.

Like uh, corduroy.

Corduroys can be made out of pure cotton as well, but look, feel, and have different tensile strengths and such than denim…

…and even denim has many variations of exact kinds of denim weaves… and nowadays, things that are marketed as ‘denim’, often contain a good bit of some kind of synthetic fiber, to give them a bit more stretch without losing too much durability.