Wordnet 3.0
NOUN (1)
1. 
 the dark negative feminine principle in Chinese dualistic cosmology; 
- Example: "the interaction of yin and yang maintains the harmony of the universe"
The Collaborative International Dictionary of English v.0.48:
Yin \Yin\, n.
   A Chinese weight of 22/3 pounds.
   [1913 Webster]
The Collaborative International Dictionary of English v.0.48:
Yin \Yin\, n. (Chinese philosophy)
   one of the two fundamental principles. See yin and yang.
   [PJC]
WordNet (r) 3.0 (2006):
yin
    n 1: the dark negative feminine principle in Chinese dualistic
         cosmology; "the interaction of yin and yang maintains the
         harmony of the universe" [ant: yang]