PEI is the acronym for polyethyleneimine, a polymerized ethyleneimine, also called aziridine, a cyclic secondary amine. The number after "PEI-", here 250, indicates the average number of monomer units - CH2-CH2-NH-. This mixture of substances is used in cosmetic formulations. It acts as viscosity regulator (increases or decreases the viscosity of cosmetic products).