The Collaborative International Dictionary of English v.0.48:

Pessary \Pes"sa*ry\ (p[e^]s"s[.a]*r[y^]), n.; pl. Pessaries.
   [L. pessarium, pessum, pessus, Gr. pesso`s: cf. F. pessaire.]
   (Med.)
   (a) An instrument or device to be introduced into and worn in
       the vagina, to support the uterus, or remedy a
       malposition.
   (b) A medicinal substance in the form of a bolus or mass,
       designed for introduction into the vagina; a vaginal
       suppository.
       [1913 Webster]

WordNet (r) 3.0 (2006):

pessary
    n 1: a contraceptive device consisting of a flexible dome-shaped
         cup made of rubber or plastic; it is filled with spermicide
         and fitted over the uterine cervix [syn: diaphragm,
         pessary, contraceptive diaphragm]