a long steep-sided depression in the ocean floor
"The sonar equipment detected a massive deep cutting through the continental shelf during the night survey."
The deep part of a lake, sea, etc.
"The scuba divers descended into the deep to explore the sunken shipwreck at the bottom of the ocean."
In plain English: A deep is the bottom part of a hole or well where you can't see the ground below.
"The deep in the ocean is filled with strange creatures."
Usage: Use "the deep" as a noun to refer specifically to the deepest, most remote section of a body of water like a lake or ocean. It is often used in phrases such as "sinking into the deep" to describe reaching that bottom area.
relatively deep or strong; affecting one deeply
"a deep breath"
"a deep sigh"
"deep concentration"
"deep emotion"
"a deep trance"
"in a deep sleep"
having great spatial extension or penetration downward or inward from an outer surface or backward or laterally or outward from a center; sometimes used in combination
"a deep well"
"a deep dive"
"deep water"
"a deep casserole"
"a deep gash"
"deep massage"
"deep pressure receptors in muscles"
"deep shelves"
"a deep closet"
"surrounded by a deep yard"
"hit the ball to deep center field"
"in deep space"
"waist-deep"
very distant in time or space
"deep in the past"
"deep in enemy territory"
"deep in the woods"
"a deep space probe"
of an obscure nature
"the new insurance policy is written without cryptic or mysterious terms"
"a deep dark secret"
"the inscrutable workings of Providence"
"in its mysterious past it encompasses all the dim origins of life"
"rituals totally mystifying to visitors from other lands"
Extending far away from a point of reference, especially downwards.
"The diver held her breath as she descended into the deep blue water below the surface."
Extending far down from the top, or surface, to the bottom, literally or figuratively.
"The diver held her breath as she swam deeper into the dark ocean trench where sunlight never reaches."
In plain English: Deep means very far down or at a great distance from the top surface.
"The deep blue ocean stretched out to the horizon."
Usage: Use "deep" to describe significant distance from a surface or bottom, such as deep water or deep snow. Avoid using it for abstract intensity unless specifically referring to the depth of feeling or thought.
Far, especially far down through something or into something, physically or figuratively.
"The diver held her breath as she swam deep beneath the surface of the ocean."
In plain English: Deep means going far down into something, like diving deep underwater.
"The rabbit dug deep into the ground to hide its food."
Usage: Use "deep" as an adverb to describe actions that extend far inward or intensely, such as diving deep into a pool or feeling deeply moved by a story. Avoid using it as a standard adjective before nouns unless referring specifically to depth measurements like deep water.
The word "deep" comes from Old English, where it originally described not just physical depth but also profound or serious qualities. It traveled into Middle English with the same broad meaning before settling on its current sense of great distance from the surface.