The formation or presence of bonds (or other attractive interactions) between two or more separate binding sites within the same ligand and a single central atom.
A molecular entity in which there is chelation (and the corresponding chemical species) is called a "chelate".
The terms bidentate (or didentate), tridentate, tetradentate... multidentate are used to indicate the number of potential binding sites of the ligand, at least two of which must be used by the ligand in forming a "chelate".
For example, the bidentate ethylenediamine forms a chelate with CuI in which both nitrogen atoms of ethylenediamine are bonded to copper. (The use of the term is often restricted to metallic central atoms.)
The phrase "separate binding sites" is intended to exclude cases such as [PtCl3(CH2=CH2)]-, ferrocene, and (benzene)tricarbonylchromium in which ethene, the cyclopentadienyl group, and benzene, respectively, are considered to present single binding sites to the respective metal atom, and which are not normally thought of as chelates (see hapto). See also cryptand.