Illustrated Glossary of Organic Chemistry

Emulsion: A suspension of microscopic liquid drops within another liquid. The drop sizes vary, so this mixture is not homogeneous, and therefore not a solution. Emulsification (a noun) and emulsify (verb) mean the process of forming an emulsion.

Mayonnaise is an emulsion of oil in water, stabilized by egg lecithin, a natural surfactant. Milk is an emulsion of fat, protein, and other things in water. Both appear opaque because the emulsified droplets are of a size range that scatter all wavelengths of white light.