Lactic acid ($\mathrm{C}{\mathrm{H}}_3\mathrm{CHOHCOOH}$)

Lactic acid is 2-hydroxypropanoic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{COOH}$. Its central carbon is a stereogenic centre, and its conjugate base is lactate.

In lactic acid fermentation, reduction of pyruvate to lactate regenerates NAD+ from NADH. That regeneration allows glycolysis to continue when the pathway cannot rely on sufficiently rapid respiratory oxidation of NADH.

Lactate is the conjugate base of lactic acid. Lactate dehydrogenase couples its interconversion with pyruvate to the NAD+/NADH redox pair.