渗透（osmosis）是水分子经半透膜扩散的现象。它由高水分子区域（即低浓度溶液）渗入低水分子区域（即高浓度溶液），直到细胞内外浓度平衡（等张）为止。水分子会经由扩散方式通过细胞膜，这样的现象，称为渗透。细胞借由渗透作用得到水分，但是也有可能因此丧失水分或得到过多的水分，例如将细胞放入浓食盐水中，由于浓食盐水中水的含量比例较细胞质低，胞内的水会不断的往胞外渗透，导致细胞脱水、萎缩；相反的，将细胞放入蒸馏水中，由于细胞内的水含量比例较蒸馏水低，外界的水分子会不断往胞内渗透，导致细胞膨胀，甚至造成破裂。

Osmosis is the spontaneous net movement of solvent molecules through a semi-permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides. It may also be used to describe a physical process in which any solvent moves across a semipermeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations. Osmosis can be made to do work.