A bright conduit of energy, along which a matter transmission is sent from one set of transport rings to another. The rings function similar to a Stargate, but over shorter distances -- within the levels of a ship, between ships, or from a ship in orbit to the surface of a planet.
Matter streams can be intercepted by a ship that also contains rings. However, if the intercepting rings are in motion in whatever is transporting them, reintegration could be fatal to the person or persons within the emerging energy stream.
"Stargate" the Movie - O'Neill and Jackson transport a bomb to Ra's ship in orbit over Abydos, destroying it.
The Devil You Know - Teal'c pilots a Tel'tak into place beneath Sokar's vessel, intercepting a matter stream containing SG-1, Martouf and Jacob Carter.
The Serpent's Venom - Jacob Carter is too late to rescue Teal'c in a matter stream exchange between Heru'ur and Apophis.