customer
n.-
1 a buyer (n.) A1 Beginnera person who buys things from a shop or business.
a person or organisation that purchases goods or services from a business.
ExampleThe shop was busy with customers buying gifts for the holiday.
ExampleThe manager implemented a new loyalty programme to encourage repeat customers and improve overall satisfaction scores.
PitfallThe shop has many clients every morning.The shop has many customers every morning.Use 'customer' for shops and restaurants; 'client' is usually for professional services like lawyers or accountants.
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2 a type of person (n.) B2 Upper Intermediate Informala person you have to deal with, often described by their personality.
a person of a specified kind, especially one that someone has to deal with or interact with.
ExampleBe careful with that dog; he can be a tough customer if he doesn't know you.
ExampleThe negotiator was known for being a cool customer, never losing his temper even during the most heated debates.
UsageUsually preceded by an adjective like 'tough', 'cool', or 'slippery' to describe a person's temperament.
From Middle English custumer, from Old French coustumier, costumier (compare modern French coutumier), from Medieval Latin custumarius (“a toll-gatherer, tax-collector”, noun), from custumarius (“pertaining to custom or customs”, adjective), from custuma (“custom, tax”). More at custom. By surface analysis, custom + -er.