Shorthand version for the Star Trek TV series "Deep Space Nine".
Also sometimes termed as "DSN", "Deep Space 9", "ST:DS9".
3 letter terminology is consistant with other shorthand references to other Star Trek TV series.
ie. TOS, TNG, DS9, VOY, ENT
I watch DS9 because unlike TNG, individuals don't always get along with other characters and so it makes it more interesting.