To buy something by paying money for it.
From Old French *pourchacier* meaning 'to seek eagerly, pursue', from *pour-* ('for') and *chacier* ('to chase'). It shifted from the idea of 'chasing after something' to 'obtaining it, especially by paying money'.
Purchase started out closer to 'pursue' than 'pay'; it was about chasing something you wanted. Today, the word sounds more formal than 'buy', which is why companies say 'purchase now' when they want to sound serious or official.
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