An Etymological Dictionary of Astronomy and Astrophysics

Math.: The property of a set in which the application of a given mathematical operation to any member of the set always has another member of the set as its result.
The intersection of all closed sets that contain a given set.

Etymology (EN): M.E., from M.Fr., from O.Fr. closure “that which encloses,” from L. clausura “lock, fortress, a closing,” from p.p. stem of claudere “to close.”

Etymology (PE): Bandeš, verbal noun of bastan “to shut, bind; to clot; to form seed buds,” from Mid.Pers. bastan/vastan “to bind, shut,” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie;”
Skt. bandh- “to bind, tie, fasten;” PIE *bhendh- “to bind,” cf. Ger. binden, E. bind.