By definition, "mortal" refers to a thing which has life, and that life will eventually end. Some species of insects only live for weeks. Some species of trees live for thousands of years.
By definition, "immortal" refers to a thing which has life, and that life will never end. An immortal being will not die and cannot be killed.
Human perspective might cause some to think that a being with very long life might be 'immortal', but that is an error. A reader's idea of "immortal" in the DresdenVerse may be based on that perspective. There are myths and legends of 'gods' who have been killed, but they had to have been mortal in the first place.
That might be true in normal worlds, but TDF has 4-5 different levels of Mortality, and some creatures fall into multiple categories (Like Wampires).
Mortal: Vanilla Mortal - Not special, not a Wizard, Changeling etc.
Mortal - Have a Mortal Soul. This include Humans, Wizards, Wampires, Sasquach/yeti/tree and mountain folk, Half-Rampires, Changeling that have not yet Chosen, etc.
Immortal:Functional Immortal - No Natural Life expectancy, Anything that eventually stops aging. Includes Wampires, Denarian Hosts, Rampires, most (all?) Fae, and most (all?) NN Spirits.
Mantled Immortals - Those creatures that, by possession of an Immortal Mantle, cannot be killed no matter how much damage they take, unless specific circumstances are met (IE conjuctions).
Cosmic Beings* - Truely Immortal Things that are unique (not a Mantle) which will not move to another host, but also cannot be truely Killed/ended. Suspected examples are Angels, Walkers, and the Sleepers.
*Cosmic beings may all have something akin to Mantles (maybe Grace) that