isn't

• idn't (eye dialect)
• in't (Northern England, informal)
• i'n (West Country, slang)
• isnae (Scotland)

From is +‎ -n't.

• (US) IPA^(key): /ˈɪzənt/, [ˈɪzn̩t]
• (UK) IPA^(key): /ˈɪzənt/, (colloquial) [ˈɪzn̩t], /ˈɪn(t)/

• Audio (US):

  (file)

• Rhymes: -ɪzənt, -ɪn, -ɪnt

isn't

1. Contraction of is not. (negative auxiliary^[1])
   • 2013 July-August, Stephen P. Lownie, David M. Pelz, “Stents to Prevent Stroke”, in American Scientist:

     As we age, the major arteries of our bodies frequently become thickened with plaque, a fatty material with an oatmeal-like consistency that builds up along the inner lining of blood vessels. The reason plaque forms isn’t entirely known, but it seems to be related to high levels of cholesterol inducing an inflammatory response, which can also attract and trap more cellular debris over time.

• innit

isn't (plural isn'ts)

1. (colloquial, nonstandard) Something or someone that is defined by the lack of the characteristic being discussed.
   • 2020 August 17, Bret Stephens, “On Being a Biden Conservative”, in The New York Times‎^[1]:

     The most obvious recommendations for Joe Biden are a succession of “isn’ts.” He isn’t Donald Trump. He isn’t Bernie Sanders. He isn’t angry, bigoted, cruel, demagogic, erratic, frightening or gross. He isn’t going to drive Americans to distraction or the country into a ditch.

• Inst., NIST, NTIS, TINs, Tsin, inst, inst., ints, nits, snit, tins