Macrocyclic polydentate compounds, usually uncharged, in which three or more coordinating ring atoms (usually oxygen or nitrogen) are or may become suitably close for easy formation of chelate complexes with metal ions or other cationic species. (Planar analogues, such as porphyrins, are excluded.) They are also known as coronands and the chelate complexes are called coronates. GTPOC (crown).
E.g. 
a crown ether (a subclass containing only oxygen as coordinating atom.)
M. Hiraoka, Crown Compounds: their Characteristics and Applications, Elsevier Science Publishers, 1982.
