transistors - Why is saturation region called so in BJT characteristics?

I understand base-collector (current) linear relationship is lost in saturation region and collector current will be lesser than \$ \beta * Ib \$ . Hence,at max, we can call such a region as non-linear region.

Saturation literally means something remaining constant and not increasing further (upon variation of another quantity). Which quantity is getting saturated in so called 'saturation region' of BJT ?

Answer

The NPN transistor consists of two diodes the B-E diode and the B-C diode.

Assuming a B-E voltage of 0.7V the B-E diode is forward biased and electrons are injected from the emitter into the base region.

For a collector voltage of, say, 1V the B-C diode is reverse biased, a field exists in the depletion zone that pulls the electrons into the collector. If the voltage at the collector terminal is reduced fewer carriers are removed from the base and for some voltage the collector is unable to remove all the carriers from the base region. The base region is saturated with carriers.

The transistor is said to be saturated.