The Collaborative International Dictionary of English v.0.48:

milkshake \milk"shake`\, milk shake \milk" shake`\(m[i^]lk"
   sh[=a]k`) n.
   A frothy drink of milk and flavoring and sometimes fruit or
   ice cream, shaken together or blended in a blender.

   Syn: milk shake, shake.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

milk shake
    n 1: frothy drink of milk and flavoring and sometimes fruit or
         ice cream [syn: milkshake, milk shake, shake]