paste
A paste is a stiff, dense ointment so packed with powder that it almost holds its own shape — think of the thick zinc paste a nurse trowels onto a sore patch of skin. It is much firmer and less greasy-feeling than an ordinary ointment because so much solid is stirred into it.
Formally, a paste is a semisolid preparation containing a high proportion (often 25–50% or more) of finely powdered solids dispersed in an ointment base. The large powder load makes it stiff, less occlusive than a plain ointment, and able to absorb fluid and exudate. It tends to stay exactly where it is placed rather than spreading, which lets it act as a protective covering confined to one spot.
These properties make pastes ideal as protective barriers and for sharply localised treatment — for instance shielding skin around a wound or applying a drug to one lesion without it migrating onto healthy skin. The same stiffness that makes them stay put also makes them hard to spread and to remove, so they are unsuited to large or hairy areas.
Zinc oxide paste applied around a leg ulcer or under a baby's nappy forms a stiff protective layer that stays in place and shields the skin from moisture and irritation.
A heavy powder load makes a paste stiff enough to stay exactly where it is placed.
A paste is essentially an ointment overloaded with powder: the extra solid makes it stiffer, less greasy, more absorbent, and far better at staying confined to one spot.