n. In electricity, in series.One behind the other; in single file: as, to drive tandem (that is, with two or more horses harnessed singly one before the other instead of abreast).n. A pair of horses (sometimes more) harnessed one before the other.n. A carriage drawn by two or more horses harnessed one before the other.n. A bicycle or tricycle on which two can ride, one in front of the other.