The Collaborative International Dictionary of English v.0.48:

Quintuplet \Quin"tu*plet\, n. [From Quintuple.]
   1. A collection or combination of five of a kind.
      [Webster 1913 Suppl.]

   2. pl. Five children born in the same labor.
      [Webster 1913 Suppl.]

   3. (Mus.) A group of five connected notes; a turn of five
      notes.
      [Webster 1913 Suppl.]

   4. A cycle having five crank shafts and adapted for five
      riders, all of whom can assist in the propulsion.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

quintuplet
    n 1: the cardinal number that is the sum of four and one [syn:
         five, 5, V, cinque, quint, quintet, fivesome,
         quintuplet, pentad, fin, Phoebe, Little Phoebe]
    2: one of five children born at the same time from the same
       pregnancy [syn: quintuplet, quint, quin]
    3: a set of five similar things considered as a unit [syn:
       quintet, quintette, quintuplet, quintuple]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "quintuplet":
   Little Phoebe, Pentateuch, Phoebe, V, cinque, fin, five,
   five bucks, five dollars, fiver, fivesome, mullet, pentachord,
   pentad, pentagon, pentagram, pentahedron, pentameter, pentapody,
   pentarchy, pentastich, pentathlon, quincunx, quint, quintet