What is it called when salt and fresh water meet?

Answer 1

Short answer:

When fresh water meets salt water and they mix, the result is brackish water.

(See related questions for a discussion of the variety of places where this occurs.)

The name of a place where salt water and fresh water mix is often an estuary which is a partially enclosed region usually near the mouth of a river. (See related link.)

More:

An estuary is a partially enclosed area where a fresh water source such as a river flows into a salt water body such as a sea or ocean. The area is termed an estuary if both ocean and salt water influxes substantially determine the nature of the body of water, hence it would normally be partially enclosed and retaining a significant portion of fresh water influx that is not immediately diluted into the seawater.

Estuaries are often both interesting biological systems and interesting geological formations.

Note: For the more general question of where fresh and salt water meet, see related questions.

Note: If one is interested in the halocline, which is a strong vertical gradient of salt concentration form sometimes at the interface of fresh water and salt water, then that is a different question.
estuary
Brackwater

Answer 2

Brackish