What is a "sound", as in (Nantucket Sound) or (Guatemala Sound)?

Answer 1

It's a long broad inlet of the ocean generally parallel to the coast.

Also can be used for a narrow strip of water connecting two larger ones.

Answer 2

I found 2 definitions for what you're talking about:
1. a relatively narrow passage of water between larger bodies of water or between the mainland and an island: Long Island Sound.
2. an inlet, arm, or recessed portion of the sea: Puget Sound.