the more or less acidulous liquid remaining after butter has been separated from milk or cream.
a similar liquid made from whole or skim milk with the addition of a bacterial culture.

Origin of buttermilk

First recorded in 1520–30; butter + milk
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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British Dictionary definitions for buttermilk



the sourish liquid remaining after the butter has been separated from milk, often used for making scones and soda bread
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for buttermilk

1520s, from butter (n.) + milk (n.). Cf. German Buttermilch. It is what remains after the butter has been churned out.

Online Etymology Dictionary, © 2010 Douglas Harper