A Cosmic Entity, also known as a Cosmic Deity is a being that exists deep in space, an Entity that’s so much more advanced than humanity can even conceive, Cosmic Entities have even been known to be able to manipulate reality in some stories and are gods in most others.
The following is some of my research on these Entities.
In the Marvel multiverse there are many Cosmic Entities, the most important of which are known as Astral Deities, Entities that are core the the Universes continuance. The Astral Deities are known as Death, Mistress Love, Lord Chaos, Master Order, Sire Hate, Infinity, Eternity and Oblivion, and many more. These Astral Deities are essentially gods with near omnipotent powers and rarely interfere with lesser creatures, they generally only act when there is a conflict that endangers the very fabric of the Universe or the Multiverse.
There are some Entities that follow strict rules to not interfere with mortal creatures, these Entities are called The Watchers and true to their name, they watch and observe the Universe, documenting and recording what they see. Due to previous actions trying to help advance another race which ended up developing nuclear weapons and destroying themselves, they now won’t help or interfere with other races. Sometimes individual Watchers will break this rule and help heroes every now and again when a threat is too high.
Quite a few Entities have also been considered as big baddies that a bunch of heroes will have to come together to fight them, notable Entities are Ego, the living planet and Galactus, a giant planet eating alien.
DC has very similar Entities but these ones prefer to interfere less with lesser mortals. They are also spilt up into several realms like Spatial, Magical and Spiritual.
H.P. Lovecraft is responisble for the Cthulhu Mythos and a genre of his own named Lovecraftian Horror. The most notable of his creations is Cthulhu himself, a giant humanoid, dragon, octopus creature that lies in a death like sleep waiting to retake the Earth when the time comes to rise.
There are many other creatures like Cthulhu that came from the stars to settle and war on Earth, these Entities are called “The Great Old Ones” and are essentially space gods. There are many stories focused around the Great Old Ones fighting in space and over Earth, popular subjects also focus around The Necronomicon, a sort of Grimoire containing magic and a history of the Great Old Ones, also the cults that worship Cthulhu and await his return.