[ iz-uh nt ]
/ ˈɪz ənt /

contraction of is not.

usage note for isn't

pronunciation note for isn't

Isn't is often pronounced [id-nt] /ˈɪd nt/ in the South Midland and Southern U.S., with [d] /d/, a stop consonant, substituting for [z] /z/, a sibilant. This substitution results from an assimilatory process in which the tip of the tongue, in anticipation of the articulatory position for the following [n] /n/, touches the upper alveolar ridge. This same process gives [wuhd-nt] /ˈwʌd nt/ for wasn't and [bid-nis] /ˈbɪd nɪs/ for business. These pronunciations are heard from speakers on all social levels. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020

British Dictionary definitions for isn't

/ (ˈɪzənt) /

contraction of

is not
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