(pronounced: Dee-men) are the physical manifestations of the souls of humans in Lyra's world. An astral projection given Akashic form. Although dæmons are a separate identity to their humans, together, both are considered to be one entity. Therefore, both the human and dæmon will die simultaneously, if one is killed.
Dæmons assume the forms of animals. During the childhood of a human, the dæmon can shapeshift into any kind of animal depending on whim or circumstances; after the child has attained maturity, their dæmon will settle into a permanent form, often reflecting the nature of their human. A person's dæmon is usually the opposite sex, although it is possible for both to have the same sex. It is a taboo for a person to touch another's dæmon; similarly, a dæmon cannot touch another person. One mentioned exception occurs when Will Parry and Lyra Belacqua touch each other's dæmon in love; this touch was described not as a violation, but as a stimulating and invigorating sensation that caused both dæmon to settle "having felt a lover's touch."
The separation between a human and their dæmon is very painful and emotionally afflicting. Excluding the dæmon belonging to witches, who could travel great distances away from each other, death would usually follow the separation due to the severe trauma. In the series, a device known as the Silver Guillotine was able to separate dæmon from their humans without resulting in death. Nevertheless, this resulted in the human becoming a zombi.
Ostensibly, dæmon can only be readily seen and acknowledged in Lyra's world. In other worlds and universes, including Will Parry's world, the dæmons are invisible and lack physical forms, but they can be taught how to see them as Mary was taught how to by Serafina Pekkala. Humans are possibly the only species who know that they possess dæmons. They are not however, the only species to have dæmons, as in the world of the dead other sentient beings such as the Mulefa and the Gallivespians as well as many other sentient creatures. Not all intelligent creatures have dæmons, such as the Panserbjørne.
In ancient Greek, "d(a)emon" simply means "spirit" and is neutral. To specifically refer to an spirit, one needed to say "cacodemon" (a word which entered English by the 12th century), or "tychodaemon" or "eudaemon" if referring to a good spirit. In our universe "cacodemon" has generally been shortened to "demon" (hence Will's confusion on first hearing the term), although "daemon" has other meanings, especially in computing. "Cacodemon" has since the late 20th century primarily referred to one of the monsters in Doom.