What is a variable-capacitance diode?

Variable-capacitance diodes are abbreviated as VCDs. They are also called varicap diodes.
Variable-capacitance diodes has a variable capacitance which is a function of the voltage between anode and cathode.
General-purpose diodes are used for rectification and signal detection, and variable-capacitance diodes are used for RF impedance matching in automatic tuners and transmitters.