straight
river
A 'strait' is a narrow body of water connecting two larger areas of water or connecting two seas.
A "peninsular" or if it connecting two larger areas an "isthmus".
isthmus.
It is a narrow strip of land connecting two larger land areas.
A narrow strip of land connecting two larger land areas usually with waterforms on either side.
An isthmus is a narrow strip of land connecting two larger land areas usually with water on either side.
Panama
You've described an Isthmus.
An isthmus.
Isthmus. As in isthmus of Panama. Glens
A land bridge. Also called a isthmus