The word classical has several meanings:

• Pertaining to the societies of the classical antiquity, ancient Greece or Rome. For example, the Greek and Latin languages, classical architecture and the cult of the Olympian gods. See also Greek mythology and Roman mythology. The study of all the foregoing may bear the name of the classics.

• In parallel to the preceding meaning, we can speak of (say) Classical Chinese or Classical Indian culture or aspects of culture when referring to a perceived apex in the development of a society or of its arts. See for example Indian classical music.

• Pertaining to the arts (painting, music, literature, etc.), the word "classical" often refers to the period of art or style known as Classicism, which has a high regard for classical antiquity. This style predominated in the 18th and early 19th centuries between the Baroque and Romantic periods. See classical music era, classical dance.

• Pertaining to music in particular, classical music refers to art music or concert music, as opposed to popular music. This includes music from all eras, including the Classical music era.

• Any mode of scientific thought prevalent at the time of some radical new innovation.

• Specifically, in physics jargon, the word classical refers to physics using the fundamental physical theories from before the advent of quantum mechanics, i.e. classical mechanics (as founded by e.g. Galilei, Newton and formalized by Hamilton and Lagrange) and classical electrodynamics (as formalized by Maxwell). Physics research making use of quantum mechanics is sometimes called modern physics.

Often, one has problems, that in principle require use of full quantum mechnical formalisms, but cannot be attacked this way due to the greater difficulty of the mathematics involved in quantum mechnics. Then one tries to get an approximate solution by assuming that for parts of the system, the results of classical physics does not differ to much from that of the quantum mechnics treatment. This is called a semiclassical approach.

Note: The theory of relativity is usually considered as a classical theory (which is one of the main open questions of contemporary physics: How to reconcile general relativity (the last word in classical physics) with quantum mechancs? -- See quantum gravity for a discussion.

• In Economics, classical economics is the school of the first economists starting with Adam Smith. Its modern successor is neoclassical economics.