InsessionIn*ses"sion\, n. [L. insessio, fr. insidere, insessum, to sit in. See Insidious.]1. The act of sitting, as in a tub or bath. "Used by way of fomentation, insession, or bath." [R.] --Holland. 2. That in which one sits, as a bathing tub. [R.] Insessions be bathing tubs half full. --Holland.