The Collaborative International Dictionary of English v.0.48:

Dosage \Dos"age\ (d[=o]s"[asl]j), n. [Cf. F. dosage. See Dose,
   v.]
   1. (Med.) The administration of medicine in doses; specif., a
      scheme or system of grading doses of medicine according to
      age, etc.
      [Webster 1913 Suppl.]

   2. The process of adding some ingredient, as to wine, to give
      flavor, character, or strength.
      [Webster 1913 Suppl.]

   3. same as dose[1]; as, 200 mg q.i.d. [WordNet sense 1]
      [PJC]

   4. the quantity of a medicine or other substance given to an
      animal, expressed as a quantity per body weight; as, use a
      dosage of 10 milligrams per kilogram
      [PJC]

   5. the quantity of radiation given to or absorbed by an
      object; as, a maximum dosage of 1 mrad per day is allowed.
      [WordNet sense 1]

   Syn: dose.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

dosage
    n 1: the quantity of an active agent (substance or radiation)
         taken in or absorbed at any one time [syn: dose,
         dosage]
    2: a measured portion of medicine taken at any one time [syn:
       dose, dosage]