NOUN (2)

1. a luminance unit equal to 1 candle per square meter measured perpendicular to the rays from the source;

2. egg or young of an insect parasitic on mammals especially a sucking louse; often attached to a hair or item of clothing;

The Collaborative International Dictionary of English v.0.48:

Nit \Nit\ (n[i^]t), n. [AS. hnitu; akin to D. neet, G. niss,
   OHG. niz; cf. Gr. koni`s, koni`dos, Icel. gnit, Sw. gnet,
   Dan. gnid, Russ. & Pol. gnida, Bohem. hnida, W. nedd.]
   (Zool.)
   The egg of a louse or other small insect.
   [1913 Webster]

   Nit grass (Bot.), a pretty annual European grass
      (Gastridium lendigerum), with small spikelets somewhat
      resembling a nit. It is also found in California and
      Chile.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nit
    n 1: a luminance unit equal to 1 candle per square meter
         measured perpendicular to the rays from the source
    2: egg or young of an insect parasitic on mammals especially a
       sucking louse; often attached to a hair or item of clothing