The Collaborative International Dictionary of English v.0.48:

Perianth \Per"i*anth\ (p[e^]r"[i^]*[a^]nth), n. [Pref. peri- +
   Gr. 'anqos flower: cf. F. p['e]rianthe.] (Bot.)
   (a) The leaves of a flower generally, especially when the
       calyx and corolla are not readily distinguished.
   (b) A saclike involucre which incloses the young fruit in
       most hepatic mosses. See Illust. of Hepatica.
       [1913 Webster]

WordNet (r) 3.0 (2006):

perianth
    n 1: collective term for the outer parts of a flower consisting
         of the calyx and corolla and enclosing the stamens and
         pistils [syn: perianth, chlamys, floral envelope,
         perigone, perigonium]