Wiktionary
top dead center
n. 1 (context automotive English) A position in the http://en.wikipedia.org/wiki/Otto%20Cycle wherein the piston is at the topmost point in its stroke. This can either be during the ''compression phase'' or the ''combustion phase''. 2 (context automotive English) The reference mark on a flywheel (and engine block) indicating the engine is in the '''top dead center''' position, when aligned.