An adjunct is something added on or attached to something else, but not an essential part of it. It's extra, supplementary, helpful but not central.

In colleges, an adjunct professor teaches classes but isn't a permanent faculty member with full benefits and responsibilities. They're brought in as needed, semester by semester. An adjunct might teach one or two courses while the regular professors handle the core teaching load.

The word appears in other contexts too. A garage is an adjunct to a house: useful and connected, but the house would still be a house without it. In grammar, an adjunct is a word or phrase that adds extra information to a sentence but isn't required for the sentence to make sense. In “She ran quickly down the street,” the words “quickly” and “down the street” are adjuncts: they tell you more about the running, but “She ran” works perfectly well on its own.

Adjunct comes from the Latin adjungere, meaning “to join to.” Unlike something integral or fundamental, an adjunct can be removed without destroying the whole. It's attached but somewhat separate, helpful but not quite necessary.