Self-ionizing solvent possessing both characteristics of Brønsted acids and bases, for example $\ce{H2O}$ and $\ce{CH3OH}$, in contrast to aprotic solvent.
Sources: Orange Book, 2nd ed., p. 30 [Terms] [Book] PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1083 [Terms] [Paper]