period; one cycle
Astronomical term for the orbital period of a celestial body, or one complete cycle of a periodic phenomenon.
One orbital period of the Earth is about 365 days.
The period of this comet is said to be 76 years.
Compound of 一 (one) and 周期 (period, cycle). The reading いっしゅうき follows regular rendaku rules for 一 + 周期.