Secular music is simply music that is not affiliated with any religious practice or tradition. The vast majority of music in the modern world is secular. Intent and lyrical content are usually more important than musical style when determining whether music is or is not secular. Historically, the balance between religious and secular music tended to tilt in the other direction, especially during the Middle Ages.
The ancient world featured both secular and religious music. Greeks, Romans, and Egyptians all made use of music for religious purposes. They also, however, all had a tradition of using music for pure entertainment and celebration. In some cases, such as the music associated with the Greek theater, these traditions blended together in ways that mixed secular and religious elements.
After the rise of Christianity, this music became somewhat less common in the European world, but the precise ratio between secular and religious music is impossible to determine. The first modern western systems of rigorous musical notation were devised by monks. These monks made written copies of religious music but generally not of medieval secular tunes, meaning that modern scholars have only snippets of information about secular music in the earlier part of the medieval period.
Secular music gradually became more important. Troubadours made use of the lute, an instrument imported to Europe during the Crusades, to compose and perform many popular secular songs, and copies of some of these were made. During the Renaissance, Europe’s focus shifted farther away from religion, and more secular tunes were composed and preserved in writing for posterity.
The Reformation saw a drop in the popularity of secular music throughout much of Europe. In time, however, secular cultural currents re-appeared, and operas, symphonies, and other pieces of familiar classical music were composed on secular themes. Religious music was still composed using the same techniques but did not dominate musical production during the Baroque era and after.
Secular music is common throughout the modern world. From jazz to opera, rock to disco, Soviet propaganda to Bollywood musical, most modern forms of popular music tend to be secular in nature. This division is not necessarily a product of the structure of modern musical forms, however. Music may be composed and performed in a predominantly secular style but designed to convey a religious message. Christian rock music, for example, draws on the musical traditions of rock and roll, but blends them with themes and lyrics drawn from Christian scripture and doctrine.