Definition of: isthmus

(is′məs, rarely isth′-) noun plural ·mus·es or ·mi (-mī)
1. A narrow body of land connecting two larger bodies.
2. Anat. A contracted passage or portion of an organ between two larger cavities or parts: the portion of the brain which joins the pons Varolii with the interbrain and hemisphere.
3. Bot. The constricted or contracted part of the cells of desmids connecting the two hemicells. [<L <Gk. isthmos narrow passage]