The substance that creates the buoyancy of a balloon or other airship; normally hydrogen, helium or hot air.

(grammar) A noun that denotes an agent that performs the action denoted by the verb from which the noun is derived, such as "rider" derived from "to ride", or "cutter" derived from "to cut".

(espionage) A spy who pretends to work for one side, when they are actually working for or passing information to the other.

a catalyst

any substance added to a food product to provide the texture of a gel

Any noxious substance intended for use in military or police operations.

A female agent provocateur; a woman who incites someone to do something, especially something incriminating.

A person who secretly disrupts a group's activities from within the group, particularly by encouraging or committing illegal acts to discredit the group or expose them to prosecution; an instigator, troublemaker.

Any food additive added to react with pectin and give the food product a stronger structure.

any substance added to flour in order to improve some aspect of its performance