Definition of key adjective from the Oxford Advanced Learner's Dictionary

  

key

 adjective
adjective
BrE BrE//kiː//
 
; NAmE NAmE//kiː//
 
 
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 [usually before noun] most important; essential synonym critical, vital the key issue/factor/point He was a key figure in the campaign. She played a key role in the dispute. ‘Caution’ is the key word in this situation. Good communication is key to our success. His contribution could be key. Synonymsmainmajor key central principal chief primeThese words all describe somebody/​something that is the largest or most important of its kind.main [only before noun] largest or most important:Be careful crossing the main road. The main thing is to remain calm.major [usually before noun] very large or important:He played a major role in setting up the system. Major is most often used after a with a singular noun, or no article with a plural noun. When it is used with the or my/​your/​his/​her/​our/​their it means ‘the largest or most important’:Our major concern here is combatting poverty. In this meaning it is only used to talk about ideas or worries that people have, not physical things, and it is also more formal than main:Be careful crossing the major road. The major thing is to remain calm.key [usually before noun] most important; essential:He was a key figure in the campaign. Key is used most frequently in business and political contexts. It can be used to talk about ideas, or the part that somebody plays in a situation, but not physical things. It is slightly more informal than major, especially when used after a noun and linking verb:Speed is key at this point.central (rather formal) most important:The central issue is that of widespread racism. Central is used in a similar way to key, but is more formal. It is most frequently used in the phrase something is central to something else. principal [only before noun] (rather formal) most important:The principal reason for this omission is lack of time. Principal is mostly used for statements of fact about which there can be no argument. To state an opinion, or to try to persuade somebody of the facts as you see them, it is more usual to use key or central:The key/​central issue here is…chief [only before noun] (rather formal) most important:Unemployment was the chief cause of poverty.prime [only before noun] (rather formal) most important; to be considered first:My prime concern is to protect my property.Patterns a/​the main/​major/​key/​central/​principal/​chief/​prime aim/​concern a/​the main/​major/​principal road/​town/​city the main/​key thing is to… to be of major/​key/​central/​prime importance Word Origin Old English cǣg, cǣge, of unknown origin.Extra examples Attitude is a key concept in social psychology. Our commitment to high quality standards is key to maintaining our reputation. Speed is key at this point. The key issue here is taxation.
See the Oxford Advanced American Dictionary entry: key