The point on the Hertzsprung-Russell diagram where stars begin to leave the main sequence as they exhaust hydrogen in their cores, used to determine the age of star clusters since more massive stars turn off first.
From 'main sequence' (the primary stellar evolutionary phase) and 'turnoff' indicating departure from this phase. The concept developed in the 1950s as stellar evolution theory matured.
The main-sequence turnoff is like reading a stellar birth certificate—by seeing which stars have just started to evolve away from the main sequence, we can determine a cluster's exact age! It's one of the most precise cosmic chronometers we have.
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