From AMS Glossary
Descriptive of a sky cover of from 0.6 to 0.9 (to the nearest tenth).
This is applied only when obscuring phenomena aloft are present, that is, not when the sky cover is composed entirely of surface-based obscuring phenomena. In aviation weather observations, a broken sky cover may be explicitly identified as thin (predominantly transparent); otherwise a predominantly opaque status is implicit. An opaque broken sky cover is the minimum requirement for a ceiling, and this is frequently termed broken ceiling.