V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

NMI
       Non-Maskable Interrupt


The Jargon File (version 4.4.7, 29 Dec 2003):

NMI
 /N?M?I/, n.

    Non-Maskable Interrupt. An IRQ 7 on the PDP-11 or 680[01234]0; the NMI
    line on an 80[1234]86. In contrast with a priority interrupt (which might
    be ignored, although that is unlikely), an NMI is never ignored. Except,
    that is, on clone boxes, where NMI is often ignored on the motherboard
    because flaky hardware can generate many spurious ones.


The Free On-line Dictionary of Computing (30 December 2018):

Non-Maskable Interrupt
NMI

   (NMI) An IRQ 7 on the PDP-11 or 680x0 or the NMI line on
   an 80x86.  In contrast with a priority interrupt (which
   might be ignored, although that is unlikely), an NMI is
   *never* ignored.

   (1994-12-13)