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

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common

adjective
ˈkɒmən
 
; ˈkɑːmən
 
 
commoner, commonest
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more common and most common are more frequent
1 happening often; existing in large numbers or in many placesJackson is a common English name.Breast cancer is the most common form of cancer among women in this country.Some birds which were once a common sight are now becoming rare.a common spelling mistakeAllergies to milk are quite common in childhood.
Opposite
uncommon
2 [usually before noun] common (to somebody/something) shared by or belonging to two or more people or by the people in a groupThey share a common interest in photography.basic features which are common to all human languagesWe are working together for a common purpose.common ownership of the landThis decision was taken for the common good(= the advantage of everyone).It is, by common consent, Scotland's prettiest coast (= everyone agrees that it is).3 [only before noun] ordinary; not unusual or specialthe common garden frogShakespeare's work was popular among the common people in his day.In most people's eyes she was nothing more than a common criminal.You'd think he'd have the common courtesy to apologize (= this would be the polite behaviour that people would expect).It's only common decency to let her know what's happening (= people would expect it).4 (British English, disapproving) typical of somebody from a low social class and not having good mannersShe thought he was very common and uneducated.
Idioms

common or garden

(British English) (North American English garden-variety) (informal) ordinary; with no special features

the common touch

the ability of a powerful or famous person to talk to and understand ordinary people

make common cause with somebody

(formal) to be united with somebody about something that you both agree on, believe in or wish to achieve
more at be common/public knowledge at knowledge