“Supplementary,” meaning “being an addition to,” is an adjective. It describes something supplied in addition to a main or existing thing, as in “supplementary reading,” “supplementary materials,” or “supplementary information.”
The word comes from “supplement,” ultimately derived from the Latin supplementum, meaning something added to complete or make up for a deficiency. The suffix “-ary” helps form adjectives meaning “related to” or “connected with.” Thus, supplementary material is material connected with a supplement—something extra that supports or completes the main content.
A common mix-up is confusing “supplementary” with “supplemental.” Both are adjectives and are usually interchangeable, although “supplementary” is especially common in British English and formal educational contexts. “Supplement” itself can be a noun, as in “a magazine supplement,” or a verb, as in “vitamins supplement the diet.” The related adverb is “supplementarily,” though it is much less common.
In mathematics, “supplementary” has a specialized use: supplementary angles are two angles whose measures add up to 180 degrees. That technical meaning still reflects the basic idea of one thing being added to another.