A gesture of celebration in which two people slap palms with the hands held above above the head.
Clearly the five are five fingers. High fives are reputed to have begun on the basketball circuit in the US in the 1979/80 season. The University of Louisville player Derek Smith claims to have invented the gesture and coined the term. That claim was recorded in the P. Dickson's 1989 Basketball Dictionary:
"The origin of the gesture and the term were claimed by Derek Smith of the University of Louisville basketball team, which won the NCAA championship in the 1979-80 season. Smith was quoted [widely] to the effect that he and two fellow Georgians on the Louisville squad, Wiley Brown and Daryl Cleveland ... [created the] high five during pre-season practice and introduced [it] to the nation in 1979."
The first records of the term in print are from 1980; for example, this from Maclean's Magazine, October 1980:
"They used to slap palms ('Gimme five, man'), but what they do now is reach high and bang hands up there ('The high five, man')."
The gesture became very widely used in the latter part of the 20th century, especially amongst black communities in the USA. It has subsequently fallen out of use somewhat and is now seen as unstylish. Other variations have taken its place, for example, the flipside or windmill. This begins with the normal high five but continues with the hands swinging to clap again low down.