supplement

(noun) textual matter that is added onto a publication; usually at the end. Synonyms: addendum, postscript.

(noun) a quantity added (e.g. to make up for a deficiency). Synonyms: supplementation.

(noun) a supplementary component that improves capability. Synonyms: accessory, appurtenance, add-on.

(verb) add as a supplement to what seems insufficient. Example: "Supplement your diet."

(verb) serve as a supplement to. Example: "Vitamins supplemented his meager diet."

(verb) add to the very end. Synonyms: append, add on, affix. Example: "He appended a glossary to his novel where he used an invented language."