Zero Age Main Sequence (ZAMS) is the time when a star first joins the main sequence on the Hertzsprung-Russell diagram (HR diagram) by burning hydrogen in its core through fusion reactions. After this time the star enters a phase of stellar evolution that is quite stable, and steadily processes Hydrogen into higher elements.
Diagram showing the evolution of the protostar
from its initial collapse to the main sequence where stars
live the majority of their lives.
Astronomers often refer to the time since ZAMS as the age of the star, although prior to ZAMS the star is referred to as a protostar and the time for the collapse can be very significant.