ointment
An ointment is a greasy, semisolid medicine you smear onto skin — think of petroleum jelly with a drug stirred into it. It feels oily and stays put, sitting on the skin like a thin protective film rather than soaking in like a watery lotion.
Technically it is a semisolid preparation, usually with little or no water, in which the drug is dissolved or dispersed in a greasy base such as petrolatum (soft paraffin) or another fatty vehicle. The base is occlusive: it forms a barrier that traps moisture and warmth in the skin, which softens the outer layer and can push a lipid-soluble drug into it, while also being soothing and emollient in its own right.
That same greasiness is the trade-off. Ointments are excellent for dry, scaly or chronic skin conditions and for delivering lipophilic drugs, but they feel sticky, are cosmetically unappealing, stain clothing, and are poorly suited to weeping or hairy areas — situations where a cream or gel is usually preferred.
Hydrocortisone ointment in a white-soft-paraffin base treats dry eczema, the occlusive vehicle hydrating the thickened skin while delivering the steroid.
A greasy base seals in moisture while carrying a lipophilic drug into the skin.
An ointment's occlusive base hydrates and softens the stratum corneum, which is often as therapeutic as the drug it carries; this is why bland petrolatum alone is a useful emollient.