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key

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noun [countable] British English pronunciation: key /kiː/ 
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singularkey
pluralkeys
  1. 1
    a small piece of metal used for opening or locking a door or a container, or for starting the engine of a vehicle

    I could hear someone turning the key in the lock.

    a bunch of keys

    house/car keys

    key to:

    Where’s the key to the back door?

  2. 2
    [usually singular] the thing that will do most to help you to achieve something
    key to:

    Proper planning is the key to success.

    The key to our strength lies in our members.

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