A MELODY is a linear succession of MUSICAL TUNES that the listener perceives as a single entity. In its most literal sense, a melody is a combination of pitch and rhythm, while more figuratively, the term can include successions of other musical elements such as tonal color. It may be considered the foreground to the background accompaniment. A line or part need not be a foreground melody.
A SONG is a composition for voice or voices, performed by SINGING. A choral or VOCAL song may be accompanied by musical instruments, or it may be unaccompanied, as in the case of a cappella songs. The lyrics (WORDS) of songs are typically of a poetic, rhyming nature, though they may be religious verses or free prose.
I don’t know why it bugs me so when people get it wrong, but it does…