Caló is the dialect of Spanish spoken by gypsies, or Roma people living in Spain. Words from Caló also have their way of sneaking into mainstream Spanish, for example the word chaval (meaning "boy") is of gypsie origin, as is the Mexican vulgar slang chingar.
Latin American slang is also sometimes called Caló, even when it doesn't have origin in the European dialect.
The name caló itself comes from a Caló word meaning "dark".