The Collaborative International Dictionary of English v.0.48:

Latency \La"ten*cy\, n. [See Latent.]
   1. The state or quality of being latent.
      [1913 Webster]

            To simplify the discussion, I shall distinguish
            three degrees of this latency.        --Sir W.
                                                  Hamilton.
      [1913 Webster]

   2. The time between a stimulus the appearance of the
      response; the time between any causal action and the first
      appearance of the effect. Called also latent period.
      [PJC]

   3. Hence: (Med.) The time between exposure to a carcinogen or
      other disease-causing agent and the appearance of the
      consequent disease.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Latent \La"tent\ (l[=a]"tent), a. [L. latens, -entis, p. pr. of
   latere to lie hid or concealed; cf. Gr. lanqa`nein, E.
   lethargy: cf. F. latent.]
   1. Not visible or apparent; hidden; concealed; secret;
      dormant; as, latent springs of action.
      [1913 Webster]

            The evils latent in the most promising contrivances
            are provided for as they arise.       --Burke.
      [1913 Webster]

   2. (Med.) Existing but not presenting symptoms; dormant or
      developing; -- of disease, especially infectious diseases;
      as, the latent phase of an infection.
      [PJC]

   Latent buds (Bot.), buds which remain undeveloped or
      dormant for a long time, but may eventually grow.

   Latent heat (Physics), that quantity of heat which
      disappears or becomes concealed in a body while producing
      some change in it other than rise of temperature, as
      fusion, evaporation, or expansion, the quantity being
      constant for each particular body and for each species of
      change; the amount of heat required to produce a change of
      phase.

   Latent period.
      (a) (Med.) The regular time in which a disease is supposed
          to be existing without manifesting itself.
      (b) (Physiol.) One of the phases in a simple muscular
          contraction, in which invisible preparatory changes
          are taking place in the nerve and muscle.
      (c) (Biol.) One of those periods or resting stages in the
          development of the ovum, in which development is
          arrested prior to renewed activity.
          [1913 Webster]

WordNet (r) 3.0 (2006):

latent period
    n 1: the time that elapses between a stimulus and the response
         to it [syn: reaction time, response time, latency,
         latent period]
    2: the time that elapses before the presence of a disease is
       manifested by symptoms