Cohort, from latin cohors, is a term which admits two different meanings. Firstly, it is a tactical unit of the Roman army who, throughout history, took different conformations. On the other hand, cohort is a series, a set, or a number.
A Roman cohort was thus an army unit which usually consisted of a single type of soldiers. A legion was formed of ten cohorts that were numbered I to X. Cohorts, in turn, were composed of three maniples (formed by two centuries). The cohort I, on the other hand, had exceptional characteristics and was composed of five double centuries.
Outside the army, there were civilian cohorts of security, such as the urban cohorts (who were in charge of the security of day) and cohorts vigilum (which devoted themselves to ensure security during the night).
As a whole or series, the concept of cohort is used in demography, epidemiology and education. A cohort is a group of people who share the same fact or event in a given period.
This means that a cohort, for example, may consist of all persons born in a City X between 1970 and 1972. Another example of a cohort is a group of students who enter primary school the same year and which, therefore, should conclude their studies together (at the same time). Thus, cohort stands for promotion.
Finally, in biology, a cohort is a group of related individuals, in this case, to a taxonomic superorder.