Definition of key noun from the Oxford Advanced American Dictionary

      

    key

     noun
    noun
    NAmE//ki//
     
     
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    tool for lock
  1. 1 a specially shaped piece of metal used for locking a door, starting a car, etc. to insert/turn the key in the lock the car keys a bunch of keys the spare key to the front door We'll have a duplicate key cut (= made). There's a special key for winding up this clock.
  2. most important thing
  3. 2[usually singular] a thing that makes you able to understand or achieve something synonym secret key (to something) The key to success is preparation. key (to doing something) The driver of the car probably holds the key to solving the crime. The key is, how long can the federal government control the inflation rate?
  4. on computer
  5. 3any of the buttons that you press to operate a computer or typewriter Press the return key to enter the information.
  6. on musical instrument
  7. 4 any of the wooden or metal parts that you press to play a piano and some other musical instruments
  8. music
  9. 5 a set of related notes, based on a particular note. Pieces of music are usually written mainly using a particular key a sonata in the key of E flat major This piece changes key many times. compare scale see also off-key
  10. answers
  11. 6a set of answers to exercises or problems Check your answers in the key at the back of the book.
  12. on map
  13. 7an explanation of the symbols used on a map or plan see also low-key
  14. Idioms
    (keep something/put something/be) under lock and key
     
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    locked up safely somewhere; in prison We keep our valuables under lock and key. I will not rest until the murderer is under lock and key.
See the Oxford Advanced Learner's Dictionary entry: key