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from "Weatherlawyer" contains these words:

Sill or cill is a supporting rail as in the bottom rail of a window or
door (threshold). It's also the name of the base beam on which timber
framed houses are built.

Yes.

But most people also use the term erroneously to refer to the board
that closes the bottm of the window opening internally.

No. That is a perfectly proper and well established use of the word.

It would be more correctto refer to the outside lip if the window is of
the type that has one as a sill is also a ledge in geology. (Or a seam
with that tendency, I think.)

Sill has a number of distinct meanings two of which are the outside
window ledge and a horizontal igneous intrusion but they do not
disqualify the other legitimate uses of the word.

--
Roger Chapman