The Collaborative International Dictionary of English v.0.48:

Roundhead \Round"head`\, n. (Eng. Hist.)
   A nickname for a Puritan. See Roundheads, the, in the
   Dictionary of Noted Names in Fiction. --Toone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

The Devil's Dictionary (1881-1906):

ROUNDHEAD, n.  A member of the Parliamentarian party in the English
civil war -- so called from his habit of wearing his hair short,
whereas his enemy, the Cavalier, wore his long.  There were other
points of difference between them, but the fashion in hair was the
fundamental cause of quarrel.  The Cavaliers were royalists because
the king, an indolent fellow, found it more convenient to let his hair
grow than to wash his neck.  This the Roundheads, who were mostly
barbers and soap-boilers, deemed an injury to trade, and the royal
neck was therefore the object of their particular indignation.
Descendants of the belligerents now wear their hair all alike, but the
fires of animosity enkindled in that ancient strife smoulder to this
day beneath the snows of British civility.