cream
A cream is the smooth, spreadable skin medicine that rubs in and disappears, leaving little or no greasy residue — closer to a hand moisturiser than to a heavy ointment. The reason it feels so different is that it is a blend of oil and water, not pure grease.
Pharmaceutically a cream is a semisolid emulsion: tiny droplets of one liquid dispersed throughout another, held stable by an emulsifying agent. An oil-in-water (o/w) cream has oil droplets in a continuous water phase, so it feels light, washes off easily and is cosmetically elegant; a water-in-oil (w/o) cream has the reverse arrangement, feels richer and more occlusive, and resists washing off. The water content makes creams vulnerable to microbial growth, so a preservative is essential.
The emulsion structure lets a cream deliver both water-soluble and oil-soluble drugs and tune how it feels and how it penetrates. The trade-off versus an ointment is durability: a cream rubs in pleasantly but offers less occlusion and a shorter contact time, so it suits moist, weeping or hairy areas where a greasy ointment would be unwelcome.
Aqueous cream and many corticosteroid creams are oil-in-water emulsions: light, non-greasy and easily washed off, which is why patients accept them for daytime use on the face.
An oil-and-water emulsion gives a cream its light, rub-in feel.
Whether a cream washes off easily or clings tells you its type: o/w creams rinse away (water is the outside), while w/o creams resist water (oil is the outside) and feel greasier.