Source : Webster's Revised Unabridged Dictionary (1913)

Roundhead \Round"head`\, n. (Eng. Hist.)
   A nickname for a Puritan. See Roundheads, the, in the
   Dictionary of Noted Names in Fiction. --Toone.

Source : WordNet®

roundhead
     n 1: a brachycephalic person
     2: a supporter of Parliament and Oliver Cromwell during the
        English Civil War