From AMS Glossary
Sea ice that has survived more than two summer melt seasons.
Such ice is typically 3 m or more thick, is less saline, and has smoother hummocks and ridges than does younger ice. Undeformed multiyear ice is distinguished by its undular surface (remnants of drained or refrozen melt ponds). Multiyear ridges are distinct from first-year ridges in that they are typically smaller, more rounded, nearly solid ice and are therefore a serious impediment to surface ships.