Supplement
Part of speech: noun
Definition: Something added, especially to make up for a deficiency. An extension to a document or publication that adds information, corrects errors or brings up to date. An additional section of a newspaper devoted to a specific subject. An angle that, when added to a given angle, makes 180; a supplementary angle. A vitamin, herbal extract, or chemical compound included with a diet to enhance muscular development.
Part of speech: verb
Definition: To provide or make a supplement to something.
Example sentence: Low levels of vitamin D in the population as a whole suggest that most people need to take a vitamin D supplement. This may be especially true for seniors, as the ability to synthesize vitamin D in the skin declines with age.
Postscript
Part of speech: noun
Definition: An addendum to a letter, added after the author's signature.