Cal. Sts. & High. Code § 257

For the purpose of this article only, and to distinguish between the terms “freeway” and “expressway,” the word “freeway” shall mean a divided arterial highway for through traffic with full control of access and with grade separations at intersections, while the word “expressway” shall mean an arterial highway for through traffic which may have partial control of access, but which may or may not be divided or have grade separations at intersections.