M.S. is a university degree you can take after your first degree, the bachelor's. The letters stand for Master of Science. It usually means you studied a science, maths, engineering, or a similar subject at the next level up — often one or two extra years, sometimes with a long paper. In American writing you often see M.S. or MS; in British writing the same kind of degree is often MSc. It is more specialised than a master's in arts (M.A.). People put it after a name on a CV: Jane Chen, M.S. In speech many just say a master's, or a master's in biology. Though actually, the letters MS on their own can mean many other things — an illness, a manuscript, a company name — so the degree meaning is the one filed here, and the rest of the sentence has to make that clear.