The Collaborative International Dictionary of English v.0.48:

nosedive \nose"dive`\, nose-dive \nose"-dive`\v. i.
   To drop with the nose or front first, as of an airplane.

   Syn: nosedive.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

nosedive
    n 1: a sudden sharp drop or rapid decline; "the stock took a
         nosedive"
    2: a steep nose-down descent by an aircraft [syn: dive, nose
       dive, nosedive]
    v 1: plunge nose first; drop with the nose or front first, of
         aircraft