a word with a special meaning used for a special occasion

a word that is created for an instance or occasion

New words that are introduced into a language to be used for a specific purpose, they are often used briefly and then drop out of usage.

From the expression, for the nonce, a word coined or used for a special circumstance or occasion only, Sidelight: Sometimes a nonce word gains acceptance in the general language, as gerrymander, which means to manipulate unfairly, such as to arbitrarily rearrange the boundaries of a political district to give one party an unfair advantage. This word was coined in 1812, when a voting district was formed with an irregular shape suggesting a resemblance to a salamander during the administration of Elbridge Gerry, then governor of Massachusetts. A word thus adopted into standard usage then ceases to be a nonce word.(Compare Neologism, Portmanteau Word, Ricochet Words)

A nonce word is a word used only "for the "—to meet a need that is not expected to recur. Quark, for example, was a nonce word appearing only in James Joyce's Finnegans Wake until Murray Gell-Mann quoted it to name a new class of subatomic particle. The use of the term nonce word in this way was apparently the work of James Murray, the influential editor of the Oxford English Dictionary.