Bilevel
From The Glossary of Digital Photography
Refers to a black-and-white image in a bitmap format. Bilevel refers to whether a given pixel should be black or white. It is often used in scanners.

ONLINE GLOSSARY OF DIGITAL PHOTOGRAPY
Refers to a black-and-white image in a bitmap format. Bilevel refers to whether a given pixel should be black or white. It is often used in scanners.