the movement of electrically charged particles, atoms, or ions, through solids, liquids, gases, or free space; the term is usually used of relatively smooth movements of electric charge through conductors, whether constant or variable. Sudden movements of charge are usually referred to by other terms, such as spark or lightning or discharge. In metallic conductors the electric current is usually due to movement of electrons through the metal. The current is measured as the rate of movement of charge per unit time, and is counted in units of amperes. As a formal definition, the direction of movement of electric current is considered as the same as the direction of movement of positive charge, or in a direction opposite to the movement of negative charge. Electric current may move constantly in a single direction, called direct current (abbreviated DC), or may move alternately in one direction and then the opposite direction, called alternating current (abbreviated AC).