main sequence
In astronomy, the band on the Hertzsprung–Russell diagram where most stars lie, fusing hydrogen in their cores.
The Sun is a main-sequence star.
principal series
In physics and mathematics, a principal series representation or related concept; a technical term used in group theory and spectral analysis.
I am researching the principal series representations of this group.
Compound of 主 (main, principal) and 系列 (series, sequence). The term is a direct calque from English 'main sequence' and 'principal series' in their respective scientific contexts.