A flying buttress is a specific form of buttress

Buttress

A buttress is an architectural structure built against or projecting from a wall which serves to support or reinforce the wall...

ing most strongly associated with Gothic church architecture

Gothic architecture

Gothic architecture is a style of architecture that flourished during the high and late medieval period. It evolved from Romanesque architecture and was succeeded by Renaissance architecture....

. The purpose of any buttress is to resist the lateral forces pushing a wall outwards (which may arise from stone vaulted

Vault (architecture)

A Vault is an architectural term for an arched form used to provide a space with a ceiling or roof. The parts of a vault exert lateral thrust that require a counter resistance. When vaults are built underground, the ground gives all the resistance required...

 ceilings or from wind-loading on roofs) by redirecting them to the ground. The characteristic of a flying buttress is that the buttress is not in contact with the wall all the way to the ground; so that the lateral forces are transmitted across an intervening space.