The Collaborative International Dictionary of English v.0.48:

Manna \Man"na\ (m[a^]n"n[.a]), n. [L., fr. Gr. ma`nna, Heb.
   m[=a]n; cf. Ar. mann, properly, gift (of heaven).]
   1. (Script.) The food supplied to the Israelites in their
      journey through the wilderness of Arabia; hence, divinely
      supplied food. --Ex. xvi. 15.
      [1913 Webster]

   2. (Bot.) A name given to lichens of the genus Lecanora,
      sometimes blown into heaps in the deserts of Arabia and
      Africa, and gathered and used as food; called also manna
      lichen.
      [1913 Webster]

   3. (Bot. & Med.) A sweetish exudation in the form of pale
      yellow friable flakes, coming from several trees and
      shrubs and used in medicine as a gentle laxative, as the
      secretion of Fraxinus Ornus, and Fraxinus
      rotundifolia, the manna ashes of Southern Europe.
      [1913 Webster]

   Note: Persian manna is the secretion of the camel's thorn
         (see Camel's thorn, under Camel); Tamarisk manna,
         that of the Tamarisk mannifera, a shrub of Western
         Asia; Australian, manna, that of certain species of
         eucalyptus; Brian[,c]on manna, that of the European
         larch.
         [1913 Webster]

   Manna insect (Zool), a scale insect (Gossyparia
      mannipara), which causes the exudation of manna from the
      Tamarix tree in Arabia.
      [1913 Webster]

WordNet (r) 3.0 (2006):

manna
    n 1: hardened sugary exudation of various trees
    2: (Old Testament) food that God gave the Israelites during the
       Exodus [syn: miraculous food, manna, manna from heaven]

The Devil's Dictionary (1881-1906):

MANNA, n.  A food miraculously given to the Israelites in the
wilderness.  When it was no longer supplied to them they settled
down and tilled the soil, fertilizing it, as a rule, with the bodies
of the original occupants.