Event horizon

Entrance and exit point of an object's journey through the Stargate where matter is disintegrated and then reintegrated. The event horizon appears to be a bright, iridescent pool of water, standing completely vertical within the ring of an active Stargate.

The Stargate is smart enough that it attempts to determine whether or not an object is attempting to pass through a connected wormhole based on way in which pressure is applied to the event horizon. A person or object sent through will feel no resistance; but if the gate is, say, activated under water it will not flood water through the wormhole.

An object or body part can also be extended through the event horizon and then pulled back, at which point it will instantly rematerialize. Holding one's arm inside the event horizon of the receiving Stargate (having not yet come all the way through the wormhole), however, will temporarily prevent both connected Stargates from shutting down.

KEY EPISODES

"Stargate" the Movie - Dr. Jackson admires the event horizon before passing through the Stargate for the first time.
Children of the Gods - Captain Carter admires the event horizon before passing through the Stargate for the first time.
The Enemy Within - Possessed by a Goa'uld, Major Charles Kawalsky is killed when the Stargate is deactivated while his head is in the event horizon.
Shades of Grey - To prevent the Stargate from shutting down so the rogue N.I.D. agents could redial and escape to another planet, Jack O'Neill keeps his arm in the event horizon after traveling through.
48 Hours - To save Teal'c's life, Carter must find a way to create an event horizon without actually establishing a wormhole to another planet.
Air, Part 3 - With Destiny about to disconnect the gate and enter F.T.L., Dr. Rush tells Eli Wallace to stick his arm into the event horizon to keep the gate open long enough for his friends to make it back.