Psycellium (or psycelium) is a psychic nervous system that allows sensates to connect with one another.
According to Yrsa, BPO possesses technology that can trace a psycellium connection by administering a drug that causes acute pain so that once a sensate is captured, they can locate any connected sensates through that shared pain. (Death Doesn't Let You Say Goodbye).There exists a drug that can temporarily cut a sensate off from the psycellium called Blockers, rendering them unable to visit or share with any other sensate.
The term itself makes reference to the Mycelium of Fungi/Mushrooms , which is known to connect individual Mushrooms miles apart via a sub-surface network. This functions as an allegory in which each sensate's individual self/consciousness is a small part of a larger, invisible, sub-surface consciousness. This allegory if referenced numerous times wherein sensates have a solitary "above" existence, and are connected "below" via the psycelium.