What is the definition of secular music?

Always break apart phrases into the Words' separate meanings. This makes it easier to fully understand. Secular means, in general, things that are not religious or spiritual. Music is vocal /and/or instrumentals, or both, that provide entertainment or expression of emotion.

So secular music is instrumental or lyrics that have nothing to do with religious ideas or views.

Example:

Though Elvis Presley was known mostly for his secular music of the rock n' roll genre, he loved singing spiritual Gospel Music.