an expression of the ratio between the concentration of products in a reaction and the concentration of reactants. It expresses the extent to which equilibrium has proceeded in either direction. Its symbol is K eq
http://www.saskschools.ca/curr_content/chem30_05/appendix/glossary.htm
(noun) Abbreviated as "K". The number obtained from the equilibrium expression of a reaction in equilibrium. If K > 1, then the concentrations of the products is greater than the reactants and the forward reaction is said to be favoured. ...
http://www.chem.ubc.ca/courseware/121_glossary/glossary.html
(chemistry) the ratio of concentrations when equilibrium is reached in a reversible reaction (when the rate of the forward reaction equals the rate of the reverse reaction)
http://wordnet.princeton.edu/perl/webwn
In chemistry, the equilibrium constant is a quantity characterizing a chemical equilibrium in a chemical reaction. It is a useful tool in determining the concentration of various reactants and products in a system where chemical equilibrium occurs.
http://en.wikipedia.org/wiki/Equilibrium constant