I have wondered what the correct musical term for the voiced melody overlaid in a piece like Jesu Joy of Man's Desire BWV147, or Sleepers Awake BWV645 is. In this piece of music (and other likes it, especially in Bach's music), it's the sung voice, sitting atop the instrumental lines. I thought I'd heard from a teacher that it was called the 'overlay', but I'm not sure this is the right term.
After researching HiDuEi's comment:
The main voice, either vocal or instrumental can be called cantus firmus. It holds the main melody. It is not important to have it as the highest element, so a cantus firmus can be arranged in the bass voice, where the upper voices accompany it. This thechnique is typical for Bach and the whole figured bass era. The other voices are referred to as counterpoint often in this style.
and suggestion the term I was looking for is cantus firmus, I believe this is the correct answer.
(HiDuEi, I've posted your comment as the answer, thanks and I hope this helps others!)