The product of reversible condensation of an aldehyde and an alcohol in which the alcoholic hydroxy group adds across the carbonyl group of the aldehyde, e.g. glucose acting as both an aldehyde and an alcohol to give the internally condensed glucopyranose; also the bond formed by this condensation. An acetal is the product formed by abstraction of the hydroxy group of a hemiacetal and the hydrogen of a second alcohol, e.g. a glucopyranoside or a polysaccharide.