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The extreme left is Day and Extreme Right is Night. In between are those Civil twilight, Nautical and Astronomical Twilight ?enter image description here

Krishna Kanth
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There are mainly 3 stages of twilight. There are defined by how many degrees below the horizon, the sun is.

  • Astronomical twilight starts when the sun is at $-18^\circ$ altitude, before sunrise. It ends when the sun is again at $-18^\circ$ altitude, but after sunset.

  • Nautical twilight starts when the sun is at $-12^\circ$ altitude, before sunrise, and ends when the sun is again at $-12^\circ$ altitude, after sunset.

  • Civil twilight starts when the sun is at $-6^\circ$ altitude, before sunrise, and ends when the sun is again at $-6^\circ$ altitude after sunset.

  • Sunrise is when the sun is at $-0.22^\circ$ altitude ( same for sunset ).

Astronomical twilight is the darkest part of twilight. An overview of these stages of twilight is here:

enter image description here

Here, C is Civil Twilight, N is Nautical Twilight and A is Astronomical Twilight.

Even in flightradar24, extreme left is day, a little bit darker is Civil Twilight, a little bit darker than civil twilight is Nautical twilight, a little bit lighter than night is Astronomical twilight. Darkest part is night.

enter image description here

Ishaan Manish
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    @Bianfable Astronomical twilight ends when the sun is at $-18^\circ$ altitude. This means that astronomical twilight is between the times when the sun is $-12^\circ$ till when the sun is $-18^\circ$. Night starts after the sun is below $-18^\circ$ altitude (after astronomical twilight).

    By terminators, do you mean twilight zones?

     – Ishaan Manish Nov 25 '21 at 09:41
  • Sorry, I misread the answer, makes sense now! I never heard the term twilight zones before, I always use terminator, but according to Wikipedia they are the same thing. I'm still not sure if this is actually the definition used by FR24 though... – Bianfable Nov 25 '21 at 09:53
  • You can use both of them, but I usually use the term twilight. That's why I thought that was better than terminator (my personal opinion, you can use either). – Ishaan Manish Nov 25 '21 at 10:55
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Stages of twilight are defined by the angle of the center of the Sun below the horizon.

  • Astronomical twilight (the darkest) begins when the center of the Sun is 18 degrees below the horizon
  • Nautical twilight begins when the Sun 12 degrees below the horizon
  • Civil twilight begins when the Sun is 6 degrees below the horizon
  • Sunrise occurs when the center of the Sun is at the horizon, ie. half of the sun is above the horizon.

At sunset stages are of course reversed. Wikipedia has more: https://en.wikipedia.org/wiki/Twilight#Definitions_by_geometry

busdriver
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    Sunrise is not when the center of the Sun is on the horizon. Sunrise is when the upper limb becomes visible. See https://en.wikipedia.org/wiki/Sunrise. Also, the original question is whether flightradar24 is showing the three zones of twilight. Your answer is for a different question: what are the definitions of twilight. – JohnHoltz Nov 24 '21 at 14:46
  • As an aside, the FAA considers civil twilight to be day and the others night. https://aviation.stackexchange.com/q/33361/8730 – TomMcW Nov 24 '21 at 19:17
  • You should probably mention "sunrise" before describing the angles; for someone thinking of sunset twilight there's confusion before realising what's meant – Chris H Nov 26 '21 at 11:39