R.C.Ch. a basin with a drain, formerly near the altar, now usually in the sacristy, for the disposal of holy water, waste water from washing altar linen, etc.
See piscina in American Heritage Dictionary 4
(pĭ-sēˈnə, -sīˈnə, -shēˈnə)
nounpl.pi·sci·nae(-nē) Ecclesiastical
A stone basin with a drain for carrying away the water used in ceremonial ablutions.