What "classical physics" refers to depends on the context. When discussing special relativity, it refers to the Newtonian physics which preceded relativity, i.e. the branches of physics based on principles developed before the rise of relativity and quantum mechanics. When discussing general relativity, it refers to the result of modifying Newtonian physics to incorporate special relativity. When discussing quantum mechanics, it refers to non-quantum physics, including special relativity, and general relativity.

Classical theory has at least two distinct meanings in Physics:

The existence of these two distinct meanings of the term can lead to confusion: special relativity is a "classical theory" in the first sense, but its predictions are more accurate than "classical theory" in the second sense.

In other contexts, "classical theory" will have other meanings—if a current accepted theory is considered to be "modern", and its introduction represented a major paradigm shift, then previous theory (or new theories based on the older paradigm) will often be referred to as "classical".