A culturally coherent entity whose members share a community of interests strong enough to hold them together beyond social differences at an individual level.

In the American Heritage Dictionary of the English Language, Fourth Edition © 2000 by Houghton Mifflin Company:

3 - A people who share common customs, origins, history, and frequently language.

I agree the notion doesn't make sense for pure liberal, i.e. one who believes in the absolute predominance of individual rights above all others.


Now, why does it matter to democracy?

Democracy assumes a large degree of mutual trust towards each other and of shared customs/values. Democracy, especially social democracy, always works best in very coherent populations: for instance, Scandinavians countries or, for quasi caricature, Switzerland at the canton level. It reduces the spectrum of disagrements and potential for serious conflicts, where minorities would feel oppressed by majoritarian rule, feel disinvested and reject democracy.

I agree it also means that it seriously reduces the scope of debates and the diversity of the body politics you can have. Successful democracies are always a bit provincial if not downright inbred.