Constant current is a term most often used in electronics

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

 to describe a system that can vary the voltage across an electronic circuit

Electronic circuit

An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...

 to maintain a constant electric current

Electric current

Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

.

When a component is indicated to be driven by a constant current, the driving circuit in essence is a current regulator and must appear to the component as a current source

Current source

A current source is an electrical or electronic device that delivers or absorbs electric current. A current source is the dual of a voltage source. The term constant-current sink is sometimes used for sources fed from a negative voltage supply...

 of suitable reliability.

An important usage of constant current power supplies are LED

LEd

LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....

s. While a high resistance series resistance is sufficient to light an LED sometimes the design needs to guard against high current. Another use is in fluorescent lamp

Fluorescent lamp

A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. A fluorescent lamp converts electrical power into useful...

s which have very dynamic electrical resistance

Electrical resistance

The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

and are optimally operated within a short range of currents.