What does classical mean in the term classical music?

'Classical music' is actually made up of a number time periods (one of the main ways works of the genre are categorized). One of these periods is known as the 'classical period'. The word 'classical' was later (ie in the 20th century) applied to the whole genre, regardless of time period.
The classical period is called 'classical' because of its concern for formal organization, a concept that observers linked to the ideals of ancient Greece (the original meaning of 'classical' refers to classical Greek). The first known use of this term for music defines 'classical' music and 'romantic' music as opposing trends. In current usage, 'classical' is more likely to be considered the opposite of 'popular' music.