Dynamis (デュナミス, Dyunamisu?), also known as akasa (アーカーシャ, Ākāsha?) to the people of Thavnair, is a concept in Final Fantasy XIV that is formally introduced in Final Fantasy XIV: Endwalker, but is ascribed to various phenomena experienced throughout the entirety of the game. Its existence is known only to a few scholars and because of its invisible and intangible nature it is rarely studied or experimented with.
Dynamis is another form of energy distinct from aether that permeates and conforms most of the physical universe, and in pure quantity is the more abundant of the two, accounting for approximately 68.3% of all known energy. However, its effect on the material realm is much weaker than aether's and is almost completely drowned out in the presence of equivalent amounts of aether. As such, dynamis only shows tangible effects when in large quantities, most notably outer space, away from comparatively aether-rich planets. The less aether a being has, the easier it is for them to use or become influenced by dynamis, with beings that can interact and manipulate dynamis being known as entelechies (エンテレケイア, Enterekeia?) by the ancients.
Dynamis is a form of energy that reacts to strong emotions, both negative and positive. Surges of strong emotion and willpower bringing strength to someone in their hour of need can be examples of dynamis in action. It is implied that Limit Breaks in certain scenarios can be empowered by dynamis.
When the entelechy hive mind collectively known as the Meteia traveled to the furthest reaches of the existing universe, where aether was extremely thin, they founded a nest of dynamis wherein they stored all negative emotions and despair they encountered through the universe and attempted to bring an end to the existence by accelerating the heat death of the universe, as well as sending waves of despair towards Etheirys. This nest, known as Ultima Thule, had no physical form as it consisted entirely of dynamis until Thancred Waters sacrificed himself and used his will to create physical embodiment of the location made of dynamis. After the Warrior of Light's journey through the realm, and the sacrifices of their companions, Ultima Thule became suffused with hope, which enabled this physical dynamis realm to maintain cohesion indefinitely.
When encountering the Twelve, G'raha Tia theorizes that Eorzeans worship of them may have served as means to grant the Twelve their current forms and powers, and that this can very well be possible through the power of dynamis. For example, Menphina explains that her loyal hound, Dalamud, appeared through the worship of the Allagan's artificial moon of the same name by the people of Eorzea, as the twin moons of the Source are objects related to the worship of Menphina.
Gameplay[]
Though dynamis itself is not used in any clear visible ways through most of the game, certain gameplay mechanics have been attributed to dynamis at times. During the final battle against the Endsinger during 
The Final Day, in order to survive her Ultimate Fate attack, a player tank has to use their Limit Break to provide the required protection from it. After this, the Endsinger wonders if it was through dynamis that the party survived. After this, as the player characters are saved from the same attack by the prayers of the Scions of the Seventh Dawn, whereupon they receive multiple stacks of the 
Prayers of Hope buff, which gives the players increased damage with each stack.
In the final quest battle part of the 
Endwalker quest, the player character begins the battle with six stacks of the 
Spark of Hope buff, which will revive the player if their health is completely depleted, with the buff described as "burning with dynamis".
During 
The Omega Protocol (Ultimate), a what-if scenario inspired by the Wandering Minstrel's songs, the Omicron Omega achieves its desire to understand what pushes heroes to victory, which is attributed to dynamis. The machine disregards its limits, interfacing with dynamis directly and fuses with Alpha, performing various attacks powered by dynamis, particularly the Run: ****mi* (Delta Version), Run: ****mi* (Sigma Version), Run: ****mi* (Omega Version), and finally Run: ****mi*, which acts as its enrage. Similarly, the player party is required to obtain the Quickening Dynamis buff which each player must stack 3 times in order to complete a phase, as well as having an instantly filling Limit Break gauge thanks to the effects of dynamis during the final phase of the fight.
Dynamis Crystals are pieces of crystalized dynamis obtainable as drops from enemies within Ultima Thule as well as in exchange for Bicolor Gemstones from certain gemstone traders.
Behind the scenes[]
The stated percentage of energy dynamis makes up is reported to be 68.3%. This alludes to the real world concept of "dark energy", a form of energy that cannot be interacted with via normal means and is believed to have noticeable effects on larger scales. It is theorized to be the reason for the expanding universe and account for 68.3% of all energy. Likewise, dynamis being used to accelerate the end of the universe is a reference to how dark energy accelerates the expansion of space.
Before the release of Endwalker, the term dynamis was first used with the Lost action item 
Dynamis Dice.
Though various instances of Limit Breaks being performed can be attributed to dynamis, not all Limit Breaks necessarily follow this, as their natures often have separate explanations as manipulations of aether or are performed by characters which are explicitly unable to manipulate dynamis. Various phenomena during the game's storylines can be attributed to dynamis but not all are made explicit, instead being attributed to the nature of mankind, determination, or manipulation of aether powered by strong emotions.
Etymology[]
Dynamis or dunamis comes from the Ancient Greek word dúnamis (δύναμις) which means "potentiality", a principle in philosophy.
Akasha or akasa comes from the Sanskrit word ākāśa (आकाश) which means "space", "sky", or "aether". This would be a parallel to the similar concept of aether.