I thought I knew what a nit was (it's one of those things I'm always picking, right?), but it turns out there's another meaning: it's a unit of brightness, equal to 1 candela per square meter. The nit page at sizes.com helpfully notes that the abbreviation is "nt," and that "A good LCD computer monitor has a brightness of about 250 nits." (That was last revised in mid-2002, so good LCD monitors may well be nittier than that by now.)
I first encountered the word in a CNET story, "Tripping the lights organic."
Btw, the search function at sizes.com doesn't seem to work very well, so if you're having a hard time finding a unit, try their index. Lots and lots of cool words there.
According to MW3, this use of "nit" derives from Latin "nitere," "to shine."
Meanwhile, it turns out there are other definitions of "nit" as well. In particular, in Australia, to "keep nit" is to keep watch.