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In ancient Rome a cohortLatin cohors, genitive cohortis - was a military unit, each legion being divided into ten cohorts. During the Roman Empire a legion comprised 5000 infantry soldiers, and so a cohort comprised 500 infantrymen. A cohort, a unit often deployed for tactical purposes, was commanded by a praefectus (prefect) and was subdivided into centuries, units of 100 men, each commanded by a centurion. In the context of non-legionary military forces (e.g., auxiliary troops (auxilia) or the Praetorian Guard) a cohort was a larger unit, comprising 1000 men.

The word cohort is also used to refer to

  • any group of warriors, as in the opening lines of the poem The Destruction of Sennacherib, by Lord Byron (1799-1824): ‘The Assyrian came down like a wolf on the fold,/And his cohorts were gleaming in purple and gold’; or
  • an associate or follower (of a particular person), as in ‘The Prime Minister and his cohorts have done all in their power to prevent a parliamentary debate on the issue’.

The word cohort is also used in a number of scientific or technical contexts. For example,

  • in biology a cohort is either (in mammalian taxonomy) a subdivision of a subclass or (in the taxonomy of plants) a subdivision of a subfamily; while
  • in statistics a cohort is a group which has a statistical feature in common, e.g., all those earning more than £100,000 per annum; all those born in a certain year; all those taking less than 30 minutes of exercise per day. Cohort is sometimes used in this way by non-statisticians, as in, e.g., ‘This year’s cohort of students seems particularly serious-minded and conscientious.’