A portal is a temporary or permanent hole in two locations that connects those two points, this can occur in space, interstellar space, intergalactic space, or even in different universes, so that they are physically adjacent, at least in a way. Portals can take many sizes; some might be so small that even single quarks cannot fit through, while others might span entire galaxy clusters. They can be of many colors, shapes, and functionalities; portals might have mass limitations restricting certain amounts of travel through them, or have a delay between entering one side and exiting the other.
Advanced civilizations might be able to achieve high enough energy densities to create portals on their own. Powerful beings can create them easily.
One limitation of portals is that they cannot be infinite in size, as that would rip existence in half, destroying everything.