The Collaborative International Dictionary of English v.0.48:

Czechs \Czechs\ (ch[e^]ks), prop. n. pl.; sing. Czech. [Named
   after their chieftain, Czech.] (Ethnol.)
   The most westerly branch of the great Slavic family of
   nations, numbering now more than 6,000,000, and found
   principally in the Czech Republic, consisting of the old
   regions of Bohemia and Moravia.