The Collaborative International Dictionary of English v.0.48:

Jettison \Jet"ti*son\ n. [See Jetsam.]
   [1913 Webster]
   1. (Mar. Law) The throwing overboard of goods from necessity,
      in order to lighten a vessel in danger of wreck.
      [1913 Webster]

   2. See Jetsam, 1.
      [1913 Webster]

WordNet (r) 3.0 (2006):

jettison
    v 1: throw away, of something encumbering
    2: throw as from an airplane

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "jettison":
   abandon, abandonment, abdicate, absentation, back out, beg off,
   boot out, booting out, bounce, cashier, cast, cast aside,
   cast away, cast off, cast out, castaway, casting away, castoff,
   cessation, chuck, chuck out, cry off, deep six, deep-six,
   defenestrate, defenestration, depart from, desuetude, detrude,
   detrusion, discard, discarding, discharge, disposal, dispose of,
   disposition, disuse, ditch, drop out, dump, dumping, eighty-six,
   eject, ejection, ejectment, eliminate, elimination, evacuate,
   evacuation, exclude, expel, expulsion, extrude, extrusion, forsake,
   forsaking, get quit of, get rid of, get shut of, give away,
   give the hook, go back on, heave out, jettisoning, jilt, junk,
   junking, kick downstairs, kick out, kicking downstairs, leave,
   leave behind, leave flat, leaving, maroon, obtrude, obtrusion,
   oust, ouster, ousting, part with, pull out, pulling out, put out,
   quit, quit cold, refuse, reject, rejectamenta, rejection,
   relegation, removal, remove, renege, riddance, say goodbye to,
   scrap, scrapping, shed, slough, stand down, take leave of,
   the boot, the bounce, throw away, throw out, throw over,
   throw overboard, throwaway, throwing out, throwing overboard,
   thrust out, toss out, toss overboard, turn out, vacate, withdraw,
   withdrawal



Bouvier's Law Dictionary, Revised 6th Ed (1856):

JETTISON, or JETSAM. The casting out of a vessel, from necessity, a part of
the lading; the thing cast out also bears the same name; it differs from
flotsam in this, that in the latter the goods float, while in the former
they sink, and remain under water; it differ; also from ligan. (q.v.)
     2. The jettison must be made for sufficient cause, and not from
groundless timidity. In must be made in a case of extremity, when the ship
is in danger of perishing by the fury of a storm, or is laboring upon rocks
or shallows, or is closely pursued by pirates or enemies.
     3. If the residue of the cargo be saved by such sacrifice, the property
saved is bound to pay a: proportion of, the loss. In ascertaining such
average. loss, the goods lost and saved are both to be valued at the price
they would have brought at the place of delivery, on the ship's arrival
there, freight, duties and other charges being deducted. Marsh. Ins. 246; 3
Kent, Com. 185 to 187; Park. Ins., 123; Poth. Chartepartie, n. 108, et suiv;
Boulay-Paty, Dr. Com. tit. 13; Pardessus, Dr. Com. n. 734; 1 Ware's R. 9.