Direct current is where at all times the voltage polarity remains constant. Unlike a.c. there is no varying cycle.
D.C. may however, particularly where it is rectified from mains a.c., contain residual a.c. superimposed or part of the voltage. This is often referred to as mains hum.
Current which moves in a single direction in a steady flow. Normal household electricity is alternating current (AC) which repeatedly reverses its direction. However, many electronics devices require DC, and therefore must convert the current into DC before using it. Diodes are used to convert AC to DC.