Two or more Diesel locomotives acting as one unit.
Consisting is a technique where several Diesel locomotives are configured to operate as one unit, and respond to one address. One engineer has control of multiple units (MU). In the days of steam the term used was Double Heading, but unlike diesels, every locomotive was fully crewed.
Digital Command Control allows prototypical operation of a multiple unit consist by one operator, who controls all the units in the consist.
For more information see MU Consisting.