Wordnet 3.0
NOUN (2)
1. 
 a factor in a given situation whose bearing and importance is not apparent; 
- Example: "I don't know what the new man will do he's still an unknown quantity"; 
2. 
 a variable whose values are solutions of an equation; 
[syn: unknown, unknown quantity]
WordNet (r) 3.0 (2006):
unknown quantity
    n 1: a factor in a given situation whose bearing and importance
         is not apparent; "I don't know what the new man will do;
         he's still an unknown quantity"
    2: a variable whose values are solutions of an equation [syn:
       unknown, unknown quantity]