word-forming element meaning "within, inside, on the inside," from Latin preposition intra "on the inside, within, in, into;" of time, "during, in the course of," related to inter "between," from PIE *en-t(e)ro-, from root *en "in." Commonly opposed to extra-, and compare inter-. The use of intra as a prefix was rare in classical Latin.
"membrane lining the abdominal cavity," early 15c., from Late Latin peritonaeum, from Greek peritonaion "abdominal membrane," literally "part stretched over," noun use of neuter of peritonaios "stretched over," from peri "around" (see peri-) + teinein "to stretch," from PIE root *ten- "to stretch." Related: Peritoneal.