The Collaborative International Dictionary of English v.0.48:

Air cell \Air" cell`\
   1. (Bot.) A cavity in the cellular tissue of plants,
      containing air only.
      [1913 Webster]

   2. (Anat.) A receptacle of air in various parts of the
      system; as, a cell or minute cavity in the walls of the
      air tubes of the lungs; the air sac of birds; a dilatation
      of the air vessels in insects.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cell \Cell\, n. [OF. celle, fr. L. cella; akin to celare to
   hide, and E. hell, helm, conceal. Cf. Hall.]
   1. A very small and close apartment, as in a prison or in a
      monastery or convent; the hut of a hermit.
      [1913 Webster]

            The heroic confessor in his cell.     --Macaulay.
      [1913 Webster]

   2. A small religious house attached to a monastery or
      convent. "Cells or dependent priories." --Milman.
      [1913 Webster]

   3. Any small cavity, or hollow place.
      [1913 Webster]

   4. (Arch.)
      (a) The space between the ribs of a vaulted roof.
      (b) Same as Cella.
          [1913 Webster]

   5. (Elec.) A jar of vessel, or a division of a compound
      vessel, for holding the exciting fluid of a battery.
      [1913 Webster]

   6. (Biol.) One of the minute elementary structures, of which
      the greater part of the various tissues and organs of
      animals and plants are composed.
      [1913 Webster]

   Note: All cells have their origin in the primary cell from
         which the organism was developed. In the lowest animal
         and vegetable forms, one single cell constitutes the
         complete individual, such being called unicelluter
         orgamisms. A typical cell is composed of a semifluid
         mass of protoplasm, more or less granular, generally
         containing in its center a nucleus which in turn
         frequently contains one or more nucleoli, the whole
         being surrounded by a thin membrane, the cell wall. In
         some cells, as in those of blood, in the am[oe]ba, and
         in embryonic cells (both vegetable and animal), there
         is no restricting cell wall, while in some of the
         unicelluliar organisms the nucleus is wholly wanting.
         See Illust. of Bipolar.
         [1913 Webster]

   Air cell. See Air cell.

   Cell development (called also cell genesis, cell
      formation, and cytogenesis), the multiplication, of
      cells by a process of reproduction under the following
      common forms; segmentation or fission, gemmation or
      budding, karyokinesis, and endogenous multiplication. See
      Segmentation, Gemmation, etc.

   Cell theory. (Biol.) See Cellular theory, under
      Cellular.
      [1913 Webster]

WordNet (r) 3.0 (2006):

air cell
    n 1: a tiny sac for holding air in the lungs; formed by the
         terminal dilation of tiny air passageways [syn: alveolus,
         air sac, air cell]