The Collaborative International Dictionary of English v.0.48:

Colymbiformes \Colymbiformes\ n.
   1. an order of birds, including the family Podicipedidae,
      which consitutes the grebes.

   Syn: Podicipitiformes, order Podicipitiformes,
        Podicipediformes, order Podicipediformes, order
        Colymbiformes.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Colymbiformes
    n 1: grebes [syn: Podicipitiformes, order Podicipitiformes,
         Podicipediformes, order Podicipediformes,
         Colymbiformes, order Colymbiformes]