The Collaborative International Dictionary of English v.0.48:

Reservoir \Res"er*voir`\ (r[e^]z"[~e]r*vw[^o]r`; 277), n. [F.
   r['e]servoir, fr. LL. reservatorium. See Reservatory.]
   1. A place where anything is kept in store; especially, a
      place where water is collected and kept for use when
      wanted, as to supply a fountain, a canal, or a city by
      means of aqueducts, or to drive a mill wheel, or the like.
      [1913 Webster]

   2. (Bot.) A small intercellular space, often containing
      resin, essential oil, or some other secreted matter.
      [1913 Webster]

   3. (Med.) a large quantity of infectious microorganisms
      resident in animals other than man, potentially capable of
      being transmitted to humans.
      [PJC]

   3. (Med.) a large quantity of infectious microorganisms or
      parasites resident in animals other than man, potentially
      capable of being transmitted to humans; especially, such
      organisms in animals where they do little or no harm to
      the host.
      [PJC]

   4. a large supply or stock of anything which may be rapidly
      put to use; a reserve.
      [PJC]

   Receiving reservoir (Water Works), a principal reservoir
      into which an aqueduct or rising main delivers water, and
      from which a distributing reservoir draws its supply.
      [1913 Webster]