suspension
Stir cocoa powder into milk and you have made a suspension: solid particles too large to dissolve, floating throughout a liquid. Pharmaceutical suspensions are the same idea — fine solid drug particles dispersed in a liquid in which they do not dissolve, so that each spoonful or drop delivers solid medicine carried by the liquid.
Suspensions are useful when a drug is poorly soluble, when it tastes bad in dissolved form, or when it is more chemically stable as a solid. They give a high dose in a swallowable liquid, are easy to give to children or patients who cannot take tablets, and can be designed for oral, topical, injectable (intramuscular depot), or ophthalmic use. The drug particles are typically in the coarse range, above about one micrometre.
Their weakness is physical instability: the solid is denser than the liquid, so particles settle over time. Good formulation slows this with a viscous vehicle, controlled particle size, and a wetting agent, and ensures that any sediment redisperses into a uniform dose with a few shakes rather than caking into a hard, unmixable plug. Hence the universal label instruction: shake well before use.
Antacid magmas such as milk of magnesia, and many paediatric antibiotic preparations reconstituted from powder, are common oral suspensions.
Shake well before use — the instruction exists because particles settle.
A controlled (flocculated) suspension is preferred over a deflocculated one for redispersibility: loose flocs resettle into a fluffy sediment that reshakes easily, while deflocculated particles can pack into a dense cake.