reciprocating engine

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. An engine whose crankshaft is turned by pistons moving up and down in a cylinder.

from Wiktionary, Creative Commons Attribution/Share-Alike License

• n. piston engine

from the GNU version of the Collaborative International Dictionary of English

• intransitive v. a steam, air, or gas engine, etc., in which the piston moves back and forth; -- in distinction from a rotary engine, in which the piston travels continuously in one direction in a circular path.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• n. an internal-combustion engine in which the crankshaft is turned by pistons moving up and down in cylinders