someone who composes music as a profession
"Mozart was a celebrated composer whose life was dedicated to writing symphonies and operas for payment."
One who composes; an author.
"The composer wrote several symphonies before gaining international fame."
In plain English: A composer is someone who writes music by creating melodies and harmonies for songs, pieces, or soundtracks.
"Beethoven was famous for being a talented composer who wrote many classic symphonies."
The word composer comes from the verb to compose with the suffix -er added to indicate a person who performs that action. It entered English directly from French and Latin without changing its core meaning of one who creates music or other works.