A character who has lost all Vitality points and become Unconscious, but who is in the process of dying because the attack that knocked them unconscious was a Called attack. From the Accelerant core rules:
"Unconscious characters that are taken down by uncalled melee or missile hits are Stable. Characters taken down by any other kind of damage, from Called melee or missile hits, from firearms, from traps, from Packets, or from anything with a Verbal become unstable."
Example: A character knocked unconscious by a strike to the shoulder would be Stable. A character knocked unconscious by a strike to the shoulder, where their opponent had called out "2 damage," would be unstable.
Some Effects can cause a character to immediately become unconscious and unstable.
It is also possible to revert to unstable - or even dead - state even though you were stable. Characters and NPCs are still able to affect unconscious characters, regardless of whether they are stable or unstable:
"If someone hits you with an attack for called damage while you are unconscious and stable you will become unstable and begin your 1 minute count."
"A Death Strike successfully delivered to your torso will kill you."
Unstable characters have 60 seconds to receive healing before they begin the Death mechanics. Once any amount of healing is received, they will wake up. There are also ways for characters to be brought out of the unstable state even though they are still unconscious and did not receive healing. From the Accelerant core rules:
"A Stabilize effect will change your condition to stable and you will begin your 5 Minute Count..."
"If someone begins to use First Aid on you, your count will be suspended until they stop the First Aid. If they call Stabilize you become stable and start your five minute count. If they do not finish the First Aid, your one minute count will continue where it was before they started using the skill."