Freely permeable to water (or other solvent) but relatively impermeable to solutes. Depending on the context, it has been used to imply impermeability to all solutes except very small uncharged molecules ( e.g., a cell membrane), or merely impermeability to very large molecules such as proteins ( e.g., a capillary membrane).
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semi·per·me·able .sem-i-'pər-mē-ə-bəl, .sem-.ī- adj partially but not freely or wholly permeable specif permeable to some usu. small molecules but not to other usu. larger particles <a \semipermeable membrane>
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semi·per·me·a·ble (sem″e-purґme-ə-bəl) permitting the passage of certain molecules and hindering that of others; see under membrane.Medical dictionary. 2011.