What does "deep sea" mean?

The deeper part of the sea or ocean in which no light penetrates.

What is a synonym for "deep sea"?

Common synonyms include: abyssal, abyssal, abyssal zone, aquatic, balneal, bassalian.

noun

noun ·Uncommon ·Advanced level

Noun

1
The deeper part of the sea or ocean in which no light penetrates. countable, uncountable
2
An open sea. (part of the sea out of sight of land) countable, uncountable
"deep sea fishing"

From Middle English depe see, equivalent to deep + sea.