This page is a simple list of definitions for common terms and phrases heard at the table/in the game world. If there are terms not listed here that you would like to see, tell me and I will get them added to the list below.
Aberrant: A type of creature that is utterly alien to most others in the world or any other plane. Beholders and mind-flayers are perhaps the two most well-known examples.
Abyss: A plane of existence reachable via portals, ritual, or the Astral Plane. It consists of an infinite level of incredibly different layers, all inhabited by various kinds of demons. Many of these layers are ruled over by a demon prince, a unique and particularly powerful demon such as Orcus or Demogorgon. Some are even ruled by gods. The 666th layer of the Abyss for example is ruled over by the evil deity Lolth.
Angel: An immortal being that serves at the behest of good-aligned deities. Angels are mythical, being as distant to mortals as the gods themselves, although in eons past, angels had been known to appear, always at a time of incredibly dire need. Angels are dived into three types: devas, planetars, and solars.
Astral Plane: A plane of existence commonly thought to exist above the world (as opposed to what would be space in the real world), this plane is essentially infinite in nature and holds a way to access all other planes of existence.
Deity: An immortal being with near-limitless power. Each resides in his or her own plane, typically of their own creation. Many are credited with the creation of some race of people. Deities hold sway over certain intangible concepts, such as Death, Knowledge, or Justice. They are able to grant power unto beings who venerate them. Deities are distant at best and have little to no direct contact with the mortal world, which has sparked some debate as to their existence.
Demon: A denizen of the Abyss. Typically chaotic in nature. They are generally incredibly violent.
Devil: A denizen of the Nine Hells. While evil, they tend to abide by their word or the word of an agreement/bargain that has been made. While not shy of violence, they can be reasoned with. Sometimes.
Domain: A domain is an aspect of reality that a deity has influence over. Different domains grant clerics access to different abilities, complimenting or augmenting their spellcasting capabilities (e.g. Life, Knowledge, and War).
Elemental: A being made up of one (or more) of the elements: fire, earth, air, water, or some permutation of them being the most common. They inhabit the Elemental Planes. Mortals can summon them into the world. They are functionally immortal, in that they do not age. They can be slain, however.
Elemental Planes: Planes of existence that are largely populated by elemental creatures, these are places of seething elements. These planes are made up of some form or another of the element for which they are named. Due to this, these places are extremely inhospitable to any being not formed of a given plane. The Elemental Planes are: Air, Water, Earth, and Fire. The elementals of corresponding elements can be found there, as well as genies of the same type. The most infamous of these planes is the Elemental Plane of Fire, largely due to a city that is known throughout the multiverse, the fabled City of Brass.
Ethereal Plane: This is a plane that is sort of overlayed upon the world, which is a weird concept to think about. Beings on this plane can see into the regular world, but not the other way around. It is very difficult for one of these planes to affect the other, though the Magic Missile spell is a notable example. Travel to this plane is relatively easy.
Fey: A denizen of the Feywild/Faerie or the World. These beings can be found on either plane though may trace their origin back to Faerie.
Feywild/Faerie: A sort of reflection of the world, but with the “wilderness” dial cranked to 11. Forests and cities in the world are sometimes reflected here, but where in the world there is a small forest, in the Feywild it may exist as a vast, flourishing primeval forest. Home of centaurs, furbolgs, unicorns, and other fey creatures.
Fiend: A denizen of the Lower Planes such as the Abyss or the Nine Hells. Typically fiends are thought of as either devils or demons, but that only refers to beings that inhabit their respective planes. Other kinds of fiends exist as well that are native to different planes — Gehana, for example, home of the Yugoloths.
Genie: A being of immense power and technically a type of elemental. Genies come in four varieties: Dao (earth), Djinn (air), Efreet (fire), and Marid (water). Some of these genies are rumored to have the ability to grant wishes to mortals. All are incredibly vain and typically not very nice.
Golem: A magical construction, typically but not always humanoid in shape, made for some purpose and animated by a ritual.
Immortal: A being that cannot die from advanced age. This is not the same as being unkillable. Most beings classified as immortal can die, though this takes more and more effort the more powerful the being.
The Lower Planes: While not really “lower” than any other plane, these planes of existence are typically thought of as being more in tune with either base (lower) emotions, or with evil (morally lower than good). This includes The Abyss and the Nine Hells as well as a handful of other planes. Beings native to these planes are generally classified as fiends.
Nine Hells: A plane of existence reachable through certain portals or via the Astral Plane. It is the home of devils and consists of Nine immense, expansive layers. In descending order, they are Avernus, Dis, Minauros, Phlegethos, Stygia, Malbolge, Maladomini, Cania, and Nessus. Movement between levels can be achieved in a variety of ways, including the River Styx. Each is ruled over by an incredibly powerful, unique devil referred to often as an archdevil. The ruler of the ninth level, Nessus, is also the de facto ruler of the Nine Hells.
Plane: A realm of existence that is typically inhabited by a variety of creatures (ex. The World, the Elemental Plane of Fire, the Abyss, the Feywild/Faerie). These planes may be reachable via a portal some other means of planar travel.
Pocket Plane: A small, artificially created realm of existence outside of the regular world, usually made for some express purpose (a hideout for example). Also sometimes referred to as a pocket dimension or a demiplane.
Portal: A sort of magical doorway used to transport people or objects across great distances and even to different planes.
Ritual: A type of spell that has a specific effect that isn’t used for combat. These can vary widely in what they are intended to achieve, from the creation of a banquet-table of nourishing food to the resurrection of a dead ally. They typically take much more time to complete than combat spells, anywhere from 5 minutes to several hours or even days, depending upon the complexity.
Underdark: A sprawling system of caves and caverns that lies a few miles below the surface of the world that is almost a world unto itself, complete with large underground lakes and even oceans. It is a strange, dark, hostile place where few surface-dwellers tend to go willingly, save for brave adventurers. The most infamous denizens of this place are the drow.