NOUN (2)

1. an innocuous or inert medication; given as a pacifier or to the control group in experiments on the efficacy of a drug;

2. (Roman Catholic Church) vespers of the office for the dead;

The Collaborative International Dictionary of English v.0.48:

Placebo \Pla*ce"bo\, n. [L., I shall please, fut. of placere to
   please.]
   1. (R. C. Ch.) The first antiphon of the vespers for the
      dead.
      [1913 Webster]

   2. (Med.) A prescription with no pharmacological activity
      given to a patient to humor or satisfy the desire for
      medical treatment.
      [1913 Webster]

   3. (Med.) a dose of a compound having no pharmacological
      activity given to a subject in a medical experiment as
      part of a control experiment in a test of the
      effectiveness of another, active pharmacological agent.
      [PJC]

   To sing placebo, to agree with one in his opinion; to be
      complaisant to. --Chaucer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

placebo
    n 1: an innocuous or inert medication; given as a pacifier or to
         the control group in experiments on the efficacy of a drug
    2: (Roman Catholic Church) vespers of the office for the dead