Definition:
Casting slip (noun) is a slip that is formulated to be used in casting poured molds. The molds used are generally plaster, which absorbs water quickly, or occasionally bisqued clay. Casting slip is deflocculated so that particles remain in suspension for a long period of time without settling. Deflocculated slip also contains less water per volume, so the cast pieces dry much faster.
Casting slips can be used for other purposes such as slip painting; however, they rarely are since ordinary slips are much easier to make.
Examples:
When a lump of solid-but-damp casting slip is held in the hand and shaken, it can become liquid and run through your fingers. Because of this property, casting slip that has dried out to become apparently solid should never be used to throw.