Suppletory means serving as a substitute or an addition, and some synonyms for this word include supplemental, complementary, supplementary, additional, and adjunct. These words are often used to describe something that adds to or completes something else. For example, a suppletory agreement might be used to add details to a contract that were not originally included. The word supplemental is often used in a similar context, such as a supplemental income source that helps to augment one's main income. Synonyms like supplementary and complementary can be used interchangeably with suppletory to indicate a similar idea of adding something extra or filling a gap.