Deep (adjective) means extending far down from the top or surface; of great intensity, strength, or seriousness; or difficult to understand. As an adverb it means far down or far in. As a noun (literary) it refers to the sea or a vast expanse such as the sky.
What Does Deep Mean?
Deep comes from Old English deop, related to Old High German tiof and Old Norse djupr, all meaning "extending far down" or "profound". The word has been in continuous use in English since at least the 9th century, making it one of the oldest and most versatile adjectives in the language.
In its most basic sense, deep describes physical measurement: a deep well, a deep lake, a deep cut. But English quickly extended this spatial idea into abstract territory. A deep sleep is one from which you are hard to wake; a deep voice has a low pitch; deep feelings are intense and long-lasting. When something is deep intellectually, it requires serious thought to understand — philosophy, literature, and science are full of "deep" ideas.
The related noun depth (formed in Middle English following the pattern of long/length and wide/width) is the standard everyday noun. The standalone noun deep — as in "creatures of the deep" — is poetic or literary. The verb deepen means to make or become deeper: "The crisis deepened overnight."
Example Sentences by Level
| Sentence | Level & Usage note |
|---|---|
| The swimming pool is very deep at this end. | A2 — physical measurement, adjective after linking verb |
| She took a deep breath before starting the exam. | B1 — fixed collocation: take a deep breath |
| He fell into a deep sleep almost immediately after lying down. | B1 — deep + noun to express intensity |
| She has a deep understanding of English grammar after years of study. | B2 — abstract, intellectual depth; adjective + noun |
| The philosopher's argument runs so deep that even seasoned academics struggle to untangle it. | C1 — adverbial use; figurative profundity |
Common Collocations
| Collocation | Example |
|---|---|
| deep breath | Take a deep breath and try again. |
| deep sleep | The medication put him into a deep sleep. |
| deep water | The boat drifted into deep water. |
| deep end | She was thrown in at the deep end on her first day. |
| deep concern | The report raised deep concern about air quality. |
| deep voice | His deep voice carried easily across the hall. |
| deep roots | The tradition has deep roots in British culture. |
| deep dive | The team did a deep dive into the quarterly figures. |
| deep blue | The sky turned a deep blue as evening fell. |
| deep down | Deep down, she knew he was right. |
Usage Notes
Deep vs Deeply
Deep as an adverb is used mainly with verbs of physical movement or position: dive deep, dig deep, buried deep in the ground. It can also appear in fixed phrases such as deep down (meaning "in one's innermost feelings").
Deeply is used with adjectives and past participles to mean "very much" or "profoundly": deeply grateful, deeply affected, deeply concerned. In most abstract or emotional contexts, deeply is the correct choice.
As a noun, the deep is literary and refers to a vast dark expanse — usually the sea or space. For ordinary prose, use the depths or the ocean instead.
Common Mistakes
Watch Out For
She is deep worried about the results.
She is deeply worried about the results. (deeply before adjectives/participles, not deep)
The lake has a ten-metre deepness.
The lake has a depth of ten metres. (use depth, not deepness, as the standard noun)
He went very deep into the argument and deep explained every point.
He went very deep into the argument and deeply explained every point. (mixing physical and abstract adverb uses)