key

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US /kiː/ 
UK /kiː/ 

Oxford Essential Dictionary

noun

1 a piece of metal that opens or closes a lock:
He turned the key and opened the door.

2 one of the parts of a computer, a piano or other musical instrument that you press with your fingers:
Pianos have black and white keys.

3 answers to questions:
Check your answers with the key at the back of the book.

Longman Dictionary of Contemporary English

key

I. key1 S3 W2 /kiː/ BrE AmE adjective [no comparative]
very important or necessary:
China’s support is key to the success of the coalition.
key factor/points/questions etc
The President makes all the key decisions on foreign policy.
key role/player/figure etc (=one with a lot of influence on a result)
The show has been hit by the departure of key personnel.
⇨ ↑low-key
• • •
COLLOCATIONS
■ nouns
a key factor (=a very important factor) A key factor in starting any business is its location.
a key element/feature/component (=a very important part) Advertising is a key element in the success of a product.
a key role Sanders played a key role in the team’s winning season.
a key area What are the key areas of economic policy?
a key issue/question/point The environment became a key issue during the election.
a key figure (=a key person) She was a key figure in the election campaign.
a key player He won’t leave key players out of the team.
a key witness (=someone who can give important information about a crime) She will be a key witness in the murder trial.
a key decision Women made most of the key decisions about how money was spent in the household.
a key word (=an important or useful word) Once you know the key words, you can make your own sentence.

Oxford Advanced Learner's Dictionary

key

key [key keys keyed keying] noun, verb, adjective   [kiː]    [kiː] 

noun
TOOL FOR LOCK
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.  

MOST IMPORTANT THING

2. usually singular a thing that makes you able to understand or achieve sth
Syn:  secret
~ (to sth) The key to success is preparation.
~ (to doing sth) The driver of the car probably holds the key to solving the crime.

(especially NAmE) The key is, how long can the federal government control the inflation rate?  

ON COMPUTER

3. any of the buttons that you press to operate a computer or typewriter

• Press the return key to enter the information.  

ON MUSICAL INSTRUMENT

 

4. any of the wooden or metal parts that you press to play a piano and some other musical instruments  

MUSIC
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  n. (7

ANSWERS

6. a set of answers to exercises or problems

• Check your answers in the key at the back of the book.  

ON MAP

7. an explanation of the symbols used on a map or plan
see also  low-key, see under lock and key at  lock  n.  
Word Origin:
Old English cǣg, cǣge, of unknown origin.  
Thesaurus:
key noun
1. C, usually sing.
The key to success is preparation.
answersolution
the key/answer/solution to sth
look for/find/provide/offer the key/an answer/a solution
2. C
Press the return key.
buttonswitchcontrol
a key/button/switch on sth
a control key/button/switch
press/hit a key/button/switch 
Example Bank:
Can we try it in a lower key?
F1 is the short-cut key for calling up help.
First-time voters could hold the key to the election result.
He fished a ring of keys out of his pocket.
Hold down the Alt key while pressing the arrow keys.
I slid the key into the lock and went in.
I'd accidentally locked the keys in my car.
I'll get another key cut so that you can have one.
Language is the key to understanding those around you.
Robbie didn't have a key for these doors.
She hides a front door key in a flowerpot.
She left a key with the neighbours/neighbors.
She must have used a key to get in.
The key changes from C major to A minor.
The key tells you what all the symbols mean.
The piece changes key in the middle.
The sound of keys jangling caught her attention.
There are two sharps in the key signature of D major.
They heard a key turn in the back door lock.
What key is it in?
You can find the symbols in the key at the bottom of the page.
You have to close it with the key.
You need a key to open the garage.
a large bunch of keys
a set of car keys
the key of G major
the key to the front door
the key to the signs and symbols
the top row of keys
His hands flew over the piano keys and beautiful sounds filled the theatre.
The key is, how long can the federal government control the inflation rate?
• The sonata was written in the key of E flat major.

Derived: key somebody to something 

Cambridge Advanced Learner's Dictionary

Cambridge Advanced Learner's Dictionary - 4th Edition
 

key / kiː / noun [ C ] (LOCK)

A1 a piece of metal that has been cut into a special shape and is used for opening or closing a lock, starting a car engine, etc.:

car/door keys

See picture key
 

key / kiː / noun [ C ] (PART)

B2 any of the set of moving parts that you press with your fingers on a computer keyboard, or musical instrument to produce letters, numbers, symbols, or musical notes

See picture key

See picture music (other)

 

key / kiː / noun [ C ] (MUSICAL NOTES)

a set of musical notes based on one particular note:

The song changes key halfway through.

the key of C minor

See picture key
 

key / kiː / noun [ C ] (SYMBOLS)

a list of the symbols used in a map or book with explanations of what they mean

See picture key
 

key / kiː / noun [ C ] (ANSWERS)

A2 a list of the answers to the questions in an exercise or test:

See the key to test 3 on page 176.

© Cambridge University Press 2013

Collins Advanced Learner’s English Dictionary

key

/ki:/
(keys, keying, keyed)

Frequency: The word is one of the 1500 most common words in English.

1.
A key is a specially shaped piece of metal that you place in a lock and turn in order to open or lock a door, or to start or stop the engine of a vehicle.
They put the key in the door and entered...
N-COUNT

2.
The keys on a computer keyboard or typewriter are the buttons that you press in order to operate it.
N-COUNT: usu pl

3.
The keys of a piano or organ are the long narrow pieces of wood or plastic that you press in order to play it.
N-COUNT: usu pl

4.
In music, a key is a scale of musical notes that starts on one specific note.
...the key of A minor.
N-VAR

5.
The key on a map or diagram or in a technical book is a list of the symbols or abbreviations used and their meanings.
You will find a key at the front of the book.
N-COUNT

6.
The key person or thing in a group is the most important one.
He is expected to be the key witness at the trial...
ADJ: ADJ n

7.
The key to a desirable situation or result is the way in which it can be achieved.
The key to success is to be ready from the start...
N-COUNT: usu N to n

Merriam-Webster's Advanced Learner's Dictionary

1key /ˈkiː/ noun, pl keys [count]
1 a : a device that is used to open a lock or start an automobile
✦The usual type of key is a small metal object that you insert into a narrow opening and turn.
• You need the key to get into the drawer.
• She turned the key and opened the door.
• house/car keys
• the ignition key of a car
• the key to the lock
- often used figuratively
• That woman holds the key to his heart.
- see also latchkey, passkey, skeleton key
b : a device that looks like a key and that is used to turn something
• a key for winding a clock
2 : something that is necessary in order to do or achieve something - usually singular
• If you want to improve your health, exercise is the key.
- usually + to
• The key to hitting the ball well is following through on your stroke.
• Hard work is the key to success.
3 a : something that provides an explanation or solution - usually + to
• the key to a riddle
b : a list of words or phrases that explain the meaning of symbols or abbreviations
• Use the key to decode the symbols.
• a pronunciation key
4 a : any one of the buttons of a computer or typewriter that you push with your fingers
• He tapped away at the keys, typing his letter.
- see picture at computer; see also function key, shift key
b : any one of the parts that you push with your fingers to play a piano or similar musical instrument
• She sat down at the piano and put her hands on the keys.
5 music : a system of musical tones based on a scale beginning on the note for which the system is named
• the key of C
• The symphony changes key from G major to E minor.
under lock and key