The Collaborative International Dictionary
Subsellium
Subsellium \Sub*sel"li*um\, n.; pl. Subsellia. [L.] (Eccl. Arch.) One of the stalls of the lower range where there are two ranges. See Illust. of Stall.
Wiktionary
subsellium
n. A projecting ledge on the stalls in a church where persons might lean whilst standing during prayers; misericord.