transsonic

/T R AE0 N S AA1 N IH0 K/
adjective
  1. 1

    Just below, or just above the speed of sound (0.8 < Ma < 1.2 approximately).

  2. 2

    Passing from subsonic to supersonic, or vice versa.

Source: https://en.wiktionary.org/wiki/transonic, https://en.wiktionary.org/wiki/transsonic