/ daɪˈɜːnl; NAmE -ˈɜːrnl/ adjective
1.
( biology ) ( of animals ) active during the day
OPP nocturnal
2.
( astronomy ) taking one day :
the diurnal rotation of the earth
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WORD ORIGIN
late Middle English (as a term in astronomy): from late Latin diurnalis , from Latin diurnus daily, from dies day.