Classical mechanics is a physical theory describing the motion of macroscopic objects. Earliest development of classical mechanics is often referred to as Newtonian mechanics. A number of modern sources do include relativistic mechanics in classical physics. Classical mechanics provides extremely accurate results when studying large objects. A number of modern sources do include relativistic mechanics in classical physics.
Largest usually classical physics refers to pre-1900 physics while modern physics refers to post-1900 physics which incorporates elements of quantum mechanics and relativity.
In the context of quantum mechanics classical theory refers to theories of physics do not use the quantization paradigm which includes classical mechanics and relativity.
Source:
[1] Wikipedia Contributors. “Classical Mechanics.” Wikipedia, Wikimedia Foundation, 13 Feb. 2021, en.wikipedia.org/wiki/Classical_mechanics. Accessed 15 Feb. 2021.
[2] 1.Wikipedia Contributors. “Classical Physics.” Wikipedia, Wikimedia Foundation, 16 Oct. 2020, en.wikipedia.org/wiki/Classical_physics. Accessed 30 Oct. 2020.