From Steven Schwartzman's book, _The Words of Mathematics_
(Mathematical Association of America):

mathematics: Latin mathematica was a plural noun, which is why
mathematics has an -s at the end even though we use it as a singular
noun. Latin had taken the word from Greek mathematikos, which in turn
was based on mathesis. That word, which was also borrowed into English
but is now archaic, meant "mental discipline" or "learning,"
especially mathematical learning. The Indo-European root is mendh-
"to learn."

John