Śīghra is basically the large circle on which the mean motion of planets was thought to take place, in the geocentric model of early Indian astronomy. This is distinguished from the small orbit (manda epicycle) that the planets were thought to travel in.
Nīlakantha describes sighra in Template:Ref-SiDar.  He writes that the sighra was in the ecliptic plane, and that the planets only went north or south from the plane thanks to their small manda epicycles. He also calls the sighra the 'fast epicycle' and the manda the 'slow epicycle'.