I-membrane engakwazi ukungena ngaphakathi iwuhlobo lolwelwesi oluvumela izinto ezithile ukuthi zidlule kulo kuyilapho luvimbela ukudlula kwezinye. Yaziwa nangokuthi i-membrane ekwazi ukungena kalula noma ulwelwesi olungena kancane. Igama elithi "semi-" libonisa ukuthi ulwelwesi alukwazi ukungena ngokuphelele, kodwa luvumela kuphela izinhlobo ezithile zama-molecule noma ama-ion ukuba zidlule, kuye ngobukhulu bawo, ukushaja, noma ezinye izakhiwo. I-Semipermeable membranes itholakala ezinhlelweni eziningi zebhayoloji nezokwenziwa, okuhlanganisa ulwelwesi lwamaseli, ulwelwesi lwe-dialysis, nolwelwesi lwe-osmosis oluhlehlayo.