Generally refers to any box-type covering separate from the jewel case and that you can slide the CD into. Usually made of thin cardboard, but some are made of plastic, like the first pressing of the FF10 soundtrack.
a box usually made of paper-covered relatively thin cardboard (not the corrugated board found in "cardboard boxes") with one end open so that one or several volumes (in this case two volumes) can be slipped into the box for protection
A slipcase is a five-sided (or sometimes four-sided) box, usually made of high-quality cardboard, into which binders, books or book sets are slipped for protection. Special editions of books are often slipcased.