What a body reports when it is being harmed or has been. Pain is information that cannot be argued with and is constantly disbelieved.
Latin poena (penalty, punishment), from Greek ποινή (poinē, payment exacted, blood-price); pain entered English as the word for punishment before it settled on the suffering a body reports.
Signal a body sends about harm, trusted here as testimony rather than argued down to a mood. What consciousness atoms name as pain, the house treats as data, not as something to be managed away.