An isthmus can be defined as  a channel of water lying between two landmasses it is dividing two land masses but isthmus is a narrow strip of land connecting two larger land areas for example isthmus of panama 
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Isthmus is a natrow strip of land which joins two big landmasses and separates two water bodies. Eg. Isthmus of Panama joins North and South America. Whereas, strait is a water body which joins to water bodies and separates two landmasses. Eg. Bening Strait joins Bening Sea and Arctic Ocean.
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