The Collaborative International Dictionary of English v.0.48:

Olympiad \O*lym"pi*ad\ ([-o]*l[i^]m"p[i^]*[a^]d), n. [L.
   olympias, -adis, Gr. 'olympia`s, 'olympia`dos, fr. 'O`lympos
   Olympus, a mountain in Macedonia: cf. F. olympiade.]
   1. (Greek Antiq.) A period of four years, by which the
      ancient Greeks reckoned time, being the interval from one
      celebration of the Olympic games to another, beginning
      with the victory of Cor[oe]bus in the foot race, which
      took place in the year 776 b. c.; as, the era of the
      olympiads.
      [1913 Webster]

   2. The quadrennial celebration of the modern Olympic games;
      as, the first Olympiad (1906). See Olympics.

   Syn: Olympic games, Olympics.
        [Webster 1913 Suppl.] Olympian

WordNet (r) 3.0 (2006):

Olympiad
    n 1: one of the four-year intervals between Olympic Games; used
         to reckon time in ancient Greece for twelve centuries
         beginning in 776 BC
    2: the modern revival of the ancient games held once every 4
       years in a selected country [syn: Olympic Games,
       Olympics, Olympiad]