LOCK.

• lock
• 1

  fastening

   [countable] a thing that keeps a door, drawer etc fastened and is usually opened with a key or by moving a small metal bar:
• I'm sorry, there isn't a lock on the bathroom door.
• The key turned stiffly in the lock.
• a bike lock
• 2

   under lock and key

• a) kept safely in a box, cupboard etc that is locked:
• Dad keeps all his liquor under lock and key.
• b) kept in a place such as a prison
• 3

   lock, stock, and barrel

  including every part of something:
• He moved the whole company, lock, stock, and barrel, to Mexico.
• 4

  hair

• a) [countable] a small number of hairs on your head that grow and hang together
• lock of
• He gently pushed a lock of hair from her eyes.
• b) 

  locks

   [plural] literary someone's hair:
• long flowing locks
• 5

  on a river etc

   [countable] a part of a canal or river that is closed off by gates so that the water level can be raised or lowered to move boats up or down a slope
• 6

  in a fight

   [countable] a hold which wrestlers use to prevent their opponent from moving:
• a head lock
• 7

  vehicle

   [uncountable and countable] British English the degree to which a vehicle's steering wheel can be turned in order to turn the vehicle
• 8

  rugby

   [countable] a playing position in the game of rugby
• 9

   a lock on something

  American English complete control of something:
• Pro football still has a lock on male viewers aged 18 to 34.
• ➔ air lock, combination lock

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.