A layer (of earth, etc.) that lies under another; substratum.
A soft floor covering that lies under a carpet.
Anything that is underlaid.
A piece of paper pasted under woodcuts, stereotype plates, etc. in a form, to bring them up to the necessary level for printing.
Lyrics; or more specifically, the way in which lyrics are assigned to musical notes.
The underlay in bar 3 is unclear in Handel's manuscript.