[For my fictional works...]
There are many aspects of divinity, but in the end, it is the ultimate expression of life, will, and creation.
Divine power is what enables sufficiently powerful deities to step into the eternal chaos-void that exists outside of and between all universes and realities, and enables a divinity to call forth a new reality.
If the powers of true creation are beyond you, then you are not a god. Will and divine power alone can make things *be*. Not called forth, nor conjured temporarily. There is no dispelling or unsummoning an object or creature made with divine power. Which is not to say that the ability to truly create makes one a god; there is more than one way to create. But no other process creates so readily, smoothly, and easily. It is a power that wants to create.
It is also what harmonizes gods and mortals and allows priests and champions to achieve power through faith and will.
Note: mortals can be 'immortal', but still be of mortal-kind.
Divinity can do all things that any other power can do. The abilities of a god can be roughly described in terms of mana or qi or cultivation or what have you, but they are never so limited or restricted as such descriptions might imply. It is true that a deity of ice and winter might not like creating fire, and might preferentially grant magic invoking the power of the cold and the dark, but there is nothing preventing it from creating fire magic if the god decides that it is the most useful thing right now.
The only restrictions of divine power are manifestations that could be considered holy or unholy.
Holy is the ultimate expression of benign intention and desires, and can only be manifested through divinities that care enough about others that they are always striving to improve the lives of mortals and give them the tools to grow beyond their current limitations.
Unholy is the ultimate expression of malignancy; the desire claim everything for oneself and the willingness to destroy all that can not be claimed. Unholy power does not require a directly divine source; its nature is to corrupt and claim, and thus is wielded by demons and devils, once mortal souls that were banished from the rest of reality because their malignancy and corruption became too great for a path forward to be found.
Many gods do not readily express their divinity in the forms of Holy or Unholy, especially if their focus is upon aspects of reality, such as elemental lords.
Perhaps ironically, the torture that fiends inflict on each other and themselves can sometimes create a fiend with just enough of the least-corrupt parts of other souls, and just enough insight to see the futility of their current path, that the fiend finds a path toward redemption and enlightenment. But for a fiend to rise is a hard and difficult path, and it is generally unwise for a mortal to make an attempt to guide a fiend onto that path, for many demons and devils are quite willing to play the part in order to lure mortals to their doom.
If one does encounter such a fiend, it is best to call upon one's deity for intercession. This is a matter that most gods are interested in, for it is the salvation of a soul, and every soul is important.