Generally the last symbol in a series of seven, acting as the point of origin. The seventh symbol "locks" the seventh chevron, which generates an active lock with another Stargate, initiating an active wormhole.
Seventh symbols can be located beneath the six other symbols on Stargate cover stones, and have at times been mistaken as part of a pictograph.
See also: point of origin
"Stargate" the Movie - Daniel Jackson describes the need for the seventh symbol in the gate address.
Stargate Origins - Catherine Langford vandalizes the seventh symbol inside the catacombs on Abydos in order to prevent the enemy from finding the full address to get back home.