Possible answer: Latin suffix from which we derive "-ism", as in "Libertarianism" or "racism" or "heroism".
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Yes, an ismus is a narrow strip of land between two water bodies that connects two larger areas of land.
The plural of isthmus is accepted as either isthmuses or isthmi.
The suffix -ism is used to form nouns denoting a distinctive practice, system, or philosophy, such as socialism or capitalism. It often signifies a belief, principle, or doctrine associated with a particular activity or ideology.
Ivanhoe, is a person that comes to mind, during the middle ages he was a leader. To get you started I will provide 7 names for you: Lee Iacocca, Janis Ian, Ice Cube, Ice-T, Billy Idol, Don Ismus, and Burl Ives.