1) A class of enzymes that catalyze the formation of a bond between two substrate molecules, coupled with the hydrolysis of a pyrophosphate bond in ATP or a similar energy donor. (Dorland, 28th ed) EC 6.

2) Major class of enzymes that catalyse the linking together of two molecules (EC class 6) for example DNA ligases that link two fragments of DNA by forming a phosphodiester bond.

3) An enzyme that catalyzes the linking together of two molecules especially by using the energy derived from the concurrent splitting off of a pyrophosphate group from a triphosphate (as ATP).