sep·tum 
(sĕptəm)
(sĕptəm)n. pl. sep·ta (-tə)
A thin partition or membrane that divides two cavities or soft masses of tissue in an organism: the nasal septum; the atrial septum of the heart.
[New Latin sēptum, from Latin saeptum, partition, from neuter past participle of saepīre, to enclose, from saepēs, fence.]