cir ‧ ca ‧ di ‧ an /sɜːˈkeɪdiən $ sɜːr-/ BrE AmE adjective [only before noun] technical
[ Date: 1900-2000 ; Language: Latin ; Origin: circa 'around, about' + dies 'day' ]
relating to a period of 24 hours, used especially when talking about changes in people’s bodies:
the body’s circadian rhythm