Compounds having the structure $\ce{R2C(OR')2}$ ($\ce{R'} \neq \ce{H}$) and thus diethers of geminal diols. Originally, the term was confined to derivatives of aldehydes (one $\ce{R = H}$), but it now applies equally to derivatives of ketones (neither $\ce{R = H}$). Mixed acetals have different $\ce{R'}$ groups.