Zero-age Main Sequence

The main sequence of the Hertzsprung-Russell diagram is the curve along which the majority of stars are located. Stars on this band are known as main-sequence stars or dwarf stars. This line is so pronounced because both the spectral type and the luminosity depend only on a star's mass (to zeroth order) as long as it is fusing hydrogen—and that is what almost all stars spend most of their "active" life doing. These main-sequence (and therefore "normal") stars are called dwarf stars not because they are unusually small. They simply have smaller radii and are less luminous than the other main type of stars, the  giant stars. "White dwarfs" are a different kind of star which are indeed small.
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The position, on the main sequence of a Hertzsprung-Russell diagram, reached by stars after they have passed through their T Tauri phase. It corresponds to a star with 75% hydrogen, 23% helium and 2% metals in its core. Nuclear fusion then gradually changes this ratio and the star evolves across the main sequence.