Lactate dehydrogenase (LDH or LD) is an enzyme found in nearly all living cells. LDH catalyzes the conversion of pyruvate to lactate and back, as it converts NAD to  NADH and back. A dehydrogenase is an enzyme that transfers a hydride from one molecule to another.
LDH exists in four distinct enzyme classes. This article is specifically about the NAD(P)-dependent -lactate dehydrogenase. Other LDHs  act on -lactate and/or are dependent on cytochrome c: D-lactate dehydrogenase (cytochrome) and L-lactate dehydrogenase (cytochrome). 
LDH is expressed extensively in body tissues, such as blood cells and heart muscle. Because it is released during tissue damage, it is a marker of common injuries and disease such as heart failure.