A transwarp aperture is an opening to a transwarp conduit which can be an entry or exit point.
The Brunali homeworld was in close proximity to a transwarp aperture, which made it a target for Borg assaults. (VOY: "Child's Play")
In 2378, USS Voyager entered aperture 823 to access transwarp conduit 9 within a Delta Quadrant-based transwarp hub. This would lead them to an exit aperture less than one light year from Earth. (VOY: "Endgame")