whilst
(wīlst, hwīlst)conj. Chiefly British While.
[Middle English whilest, alteration of whiles, whiles; see whiles.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
whilst
(waɪlst) [C13: from whiles + -t as in amidst]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
whilst
(ʰwaɪlst, waɪlst)
conj. [1325–75; Middle English
whilest=
whiles + parasitic
-t as in
amongst, amidst]
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.
whilst
Whilst is a formal word which has the same meaning as 'while'. It is used in both time clauses and concessive clauses.
Her sister had fallen whilst walking in her sleep at night.
Raspberries have a matt, spongy surface whilst blackberries have a taut, shiny skin.
You do not use whilst in conversation, and it is not used in American English..
Collins COBUILD English Usage © HarperCollins Publishers 1992, 2004, 2011, 2012