1 a : having a large distance to the bottom from the surface or highest point ▪ We walked in the deep snow. ▪ a deep well/pool/hole ▪ a deep valley between the mountains ▪ The water is deepest in the middle of the lake. ▪ She's afraid of swimming in deep water. ▪ a plant with deep roots —often used figuratively ▪ The tradition has deep roots in our culture. —opposite shallow b : going far inward from the outside or the front edge of something ▪ The house has lots of deep closets. ▪ deep shelves ▪ She has a small but deep wound on her arm. —opposite shallow c : located far inside something —usually + in or within ▪ The animals live deep in/within the forest/jungle/mountains, far from any people. ▪ His hands were deep in his pockets. ▪ The sound came from deep within his throat. —often used figuratively ▪ The memories were hidden deep in his mind. [=he had not thought about the memories in a long time] ▪ I knew deep in my heart [=I believed very strongly] that we would succeed.
2 not used before a noun : having a specified measurement downward, inward, or backward ▪ This enormous canyon is over a mile deep. ▪ The shelves are 10 inches deep. ▪ We walked through knee-deep snow [=snow as high as our knees] to get to school. ▪ The basement was waist-deep in water. [=the top of the water that filled the basement was as high as a person's waist] ▪ He stepped into an ankle-deep puddle of mud. —see also skin-deep
3 a : located near the outside edges of an area ▪ (baseball) He hit a fly ball to deep right field. b : hit, thrown, or kicked a long distance ▪ The quarterback threw a deep [=long] pass into the end zone.
4 : done by taking in or breathing out a large amount of air ▪ Take a deep breath and try to relax. ▪ a deep sigh
5 : low in sound or musical pitch ▪ I could hear my father's deep voice from down the street. ▪ a deep bass line —opposite high
6 : having a dark, strong color ▪ She painted white clouds in a deep blue sky. ▪ The walls were a deep red. ▪ deep rich colors —opposite light, pale
7 : very intelligent and serious but complex or difficult to understand ▪ She's always been a deep [=profound] thinker. = She's very deep. ▪ This book is far too deep for me. ▪ He has some very deep thoughts on the issue. ▪ a deep discussion on the meaning of life
8 : full of mystery ▪ The main character has a deep, dark secret that is revealed at the end of the movie.
9 a : completely involved in an activity : concentrating and giving all of your attention to something —+ in ▪ He was so deep in thought that he didn't hear us come in. ▪ We spent the next few hours deep in conversation. b : affected by something in a very serious way —+ in ▪ They found themselves deep in debt. [=they had a lot of debt; they owed a large amount of money]
10 : very bad, serious, or severe ▪ The country's economy fell into a deep depression/recession. ▪ There are still deep divisions within the group. [=people in the group have very different opinions and can't agree] ▪ Many people here live in deep poverty. [=many people are very poor] ▪ The entire family was in deep shock after hearing about the accident. ▪ I got in deep trouble with my parents for staying out too late.
11 : very strongly felt ▪ I offered them my deepest sympathy. ▪ They shared a deep [=profound] concern for the environment. ▪ She felt a deep [=heartfelt] connection with the culture. ▪ a deep sense of happiness and well-being ▪ the deep emotional bond between parent and child ▪ deep feelings of loss ▪ The book made a deep impression on his young mind.
12 : full, complete, or thorough ▪ She has a deep understanding of the company's needs. ◊If you are in a deep sleep, you are thoroughly asleep and it is hard to wake you up. ▪ He fell into a deep sleep and didn't wake up until the afternoon.
13 US : going down to a very low price, level, etc. ▪ a deep reduction in price ▪ The store offered deep discounts during the grand opening sale.
14 US sports : having many good players ▪ The team is very deep this year. ▪ The baseball team has a deep bullpen.