A narrow piece of land separating two water bodies and connected to land is called:

An isthmus is precisely defined as a narrow strip of land that connects two larger landmasses while separating two bodies of water. A classic example is the Isthmus of Panama, connecting North and South America and separating the Atlantic and Pacific Oceans.

• A strait is the opposite: a narrow passage of water connecting two larger bodies of water.
• A bay is a broad inlet of the sea where the land curves inward.
• A weir is a low dam built across a river to alter its flow.

Therefore, "Isthmus" is the only term that accurately describes the geographical feature mentioned, highlighting a key concept in physical geography.