noun

• (chemistry) A molecule or moiety (ion, functional group, or radical) that binds to another chemical entity to form a larger complex; as, especially:
• (coordination chemistry) Such an entity that binds to a central metal atom to form a coordination complex.
• (biochemistry, physiology, pharmacology, medicine) Such an entity that forms a complex with a biomolecule to serve a biological purpose.
• (typography) A letter that orthography requires or allows to be ligated with one or more other letters to form a ligature, such as a in æ or o in œ.