What was “new math”?

A google search is pointless. I did get one definition – something about “set theory” but that didn’t help. Mostly google gives you “help for NEW MATH teachers” and “What’s NEW – MATH department buys books”. What was this plague of the 1960s?

Staff Report: What exactly was the “new math”?

There was a thread on this very topic last year:

"New Math"