Zero-age main sequence (ZAMS, also called standard main sequence)
is a term for the main-sequence relationship
(absolute magnitude to effective temperature)
specifically for those stars that have just begun hydrogen fusion.
As plotted on an H-R diagram, it is a subset of the main sequence.
The term terminal-age main sequence (TAMS) is also used,
to indicate a star that is ceasing hydrogen fusion in the core,
ready to evolve into its post-main-sequence stages.