The Collaborative International Dictionary of English v.0.48:

Sepal \Se"pal\, n. [NL. sepalum, formed in imitation of NL.
   petalum, petal, to denote one of the divisions of the calyx:
   cf. F. s['e]pale.] (Bot.)
   A leaf or division of the calyx.
   [1913 Webster]

   Note: When the calyx consists of but one part, it is said to
         be monosepalous; when of two parts, it is said to be
         disepalous; when of a variable and indefinite number of
         parts, it is said to be polysepalous; when of several
         parts united, it is properly called gamosepalous.
         [1913 Webster]

WordNet (r) 3.0 (2006):

sepal
    n 1: one of the green parts that form the calyx of a flower