An Inter-Dimensional Portal is a portal, door or gateway that connects one area of the Multiverse to another, created through either science or the mystic arts.
The Masters of the Mystic Arts have the ability of opening these portals using Sling Rings and much practice. Other people have also been known to create these portals using either science or other types of magic.
The Masters of Mystic the Arts
Discovery of the Multiverse
The Sorcerer Supreme Agamotto became the first known human to learn about other dimensions. He dedicated his life to protecting the Earth realm from extra-dimensional threats using magic. The Masters of the Mystic Arts also discovered a way to open portals using Sling Rings.
Discovery of the Darkforce Dimension
During the 1940s, Humans began experimenting with nuclear energy; during one such experiment performed by Isodyne Energy, a rift was created where a substance dubbed Zero Matter entered this dimension. Eventually, the scientists Whitney Frost and Jason Wilkes became infected with the substance. Frost actively sought more of the Zero Matter and reopened the rift; however Wilkes entered the opening. Upon his return, he described his experience in another dimension outside the Earth realm.
The Darkhold is a book that allows the reader to build the things they want the most. The book is made of Dark Matter. Using this book the Life-Model Decoy Aida was able to build an Inter-Dimensional Gate. This gate was then used to rescue agents Phil Coulson and Leo Fitz who were trapped in another dimension. 
Ghost Rider's Return to Hell
When Thor and Loki came to Earth in order to find their father, Odin, Doctor Strange sensed Loki's presence and summoned a portal which sent him to a dimension where he would fall non-stop. Soon after, Doctor Strange opened another portal for Thor and Loki to travel through, in order to get to Norway from New York City. 
- Wormholes created by either the Bifrost or other means like the Monolith or the Tesseract share a lot of functions with some of the inter-dimensional gates listed here. The difference is that a wormhole can only create a "short-cut" between places inside the universe and not to other dimensions.
- Inter-dimensional travel is also possible through other means beside portals. For instance, Ant-Man was able to travel to another dimension without using a portal; he shrunk until he caught a glimpse of the Quantum Realm. The Spirit of Vengeance is also capable of travelling between dimensions without the need for a portal.
- ↑ Doctor Strange
- ↑ Agent Carter: 2.02: A View in the Dark
- ↑ Agent Carter: 2.08: The Edge of Mystery
- ↑ Agent Carter: 2.10: Hollywood Ending
- ↑ Guardians of the Galaxy Vol. 2
- ↑ Agents of S.H.I.E.L.D.: 4.7: Deals With Our Devils
- ↑ Agents of S.H.I.E.L.D.: 4.21: The Return
- ↑ Agents of S.H.I.E.L.D.: 4.22: World's End
- ↑ Thor: Ragnarok