Classical physics refers to theories of physics that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be "modern," and its introduction represented a major paradigm shift, then the previous theories, or new theories based on the older paradigm, will often be referred to as belonging to the realm of "classical" physics.
As such, the definition of a classical theory depends on context. Classical physical concepts are often used when modern theories are unnecessarily complex for a particular situation.
Classical theory has at least two distinct meanings in physics. In the context of quantum mechanics, classical theory refers to theories of physics that do not use the quantisation paradigm, which includes classical mechanics and relativity. Likewise, classical field theories, such as general relativity and classical electromagnetism, are those that do not use quantum mechanics. Classical theories are those that obey Galilean relativity, while general and special relativity use a different framework, but all are part of classical physics.
Among the branches of theory included in classical physics are:
Newton's laws of motion
Classical Lagrangian and Hamiltonian formalisms
Classical electrodynamics (Maxwell's Equations)
Special relativity and general relativity
Classical chaos theory and nonlinear dynamics Classical Physics, Wikipedia
Figure 3.19 - The Observable forms from the Non-observable
Figure 3.20 - The Observable Disappears into the Non-observable
Table of Cause and Effect Dualities