Etymologophile (hope I spelt that right), I don't know what it is called in plainchant notation (neume), but I think in general it is just an accidental which lowers rather than raises the note. I only used Plainsong as an example because it is the oldest form of music which is written down as far as I know, and if you look up Plainsong on Wikipedia, at the top of the page is a Kyrie in neume notation which has flats as accidentals.

I don't know of a notation for lowering by a quarter tone though.