"Crystal set" is a term that refers to a type of radio receiver that operates only on the power of the radio waves it receives, without any external source of power. Some of the synonyms for "crystal set" include "cat's whisker radio," "foxhole radio," "amateur radio," and "homebrew radio." "Cat's whisker radio" is a name for early crystal radios, as they used a fine wire (a cat's whisker) to make a delicate and finicky contact with the crystal. "Foxhole radio" was named after its use during World War II, where soldiers would build makeshift radios using scavenged parts. "Amateur radio" and "homebrew radio" both refer to radio enthusiasts building and using their own equipment.