In its traditional meaning, a gateway (or doorway) is a means of access or entry to a place, often one that can be closed and opened.
In the context of exopolitics, ufology and extraterrestrial contact, a gateway is an access point to another reality, which is different from a persons current location on Earth. This different reality can be a different location on Earth, a different world, a different realm, another dimension, or alternate and/or parallel universes.
Often, the words portal and doorway are often used in the same sense.
Such gateways can be natural or artificial. Xendras are examples of artificial gateways.
As a symbol, gateways stand for transition points, often new beginnings.