What is "triple-expansion steam engine"

Triple \Tri"ple\, a. [L. triplus; tri- (see Tri-) + -plus, as in duplus double: cf. F. triple. See Double, and cf. Treble.]

1. Consisting of three united; multiplied by three; threefold; as, a triple knot; a triple tie.

   By thy triple shape as thou art seen.
   --Dryden.

2. Three times repeated; treble. See Treble.

3. One of three; third. [Obs.]
   --Shak.

   Triple crown, the crown, or tiara, of the pope. See Tiara, 2.

   Triple-expansion steam engine, a compound steam engine in which the same steam performs work in three cylinders successively.

   Triple measure (Mus.), a measure of tree beats of which first only is accented.

   Triple ratio (Math.), a ratio which is equal to 3.

   Triple salt (Chem.), a salt containing three distinct basic atoms as radicals; thus, microcosmic salt is a triple salt.

   Triple star (Astron.), a system of three stars in close proximity.

   Triple time (Mus.), that time in which each measure is divided into three equal parts.

   Triple valve, in an automatic air brake for railroad cars, the valve under each car, by means of which the brake is controlled by a change of pressure in the air pipe leading from the locomotive.