What does cohort mean?
A cohort is a group of people who share a defining characteristic and are considered together, most commonly in research and statistics. Demographers speak of a birth cohort — everyone born in a given year — while epidemiologists run cohort studies that follow groups over decades to trace how exposures influence health outcomes. The word also has an older, informal sense: one's associates, allies, or accomplices, usually in the plural as "her political cohorts." Its pedigree is martial rather than medical — in ancient Rome, a cohort was a tactical unit of the legion, several hundred strong, so the modern sense of people bound together has deep roots. Today the word carries a scholarly, slightly technical flavor, prized for its precision where vaguer terms like "group" would blur distinctions of age, experience, or circumstance. For anyone reading social science or public-health reporting, "cohort" is indispensable vocabulary.
A group of people sharing a common demographic characteristic, such as year of birth, studied or considered together over time.
"The longitudinal study compared outcomes across each five-year birth cohort."
Standard in epidemiology, sociology, and demography; often paired with "study" or "effect."
An associate, companion, or supporter; frequently in the plural to mean a band of collaborators or accomplices.
"He and his cohorts spent the summer restoring the old theater."
Historically, one of the ten units of a Roman legion, consisting of several centuries of infantry.
"The Tenth Legion's first cohort held the line against the cavalry charge."
In biological classification, a rank between subclass and order.
"The species belongs to the same cohort within its class."
The standard plural adds -s without change to the base form. In informal use, "cohorts" almost always refers to companions or associates rather than multiple research groups.
"Several cohorts of students have passed through the program since it began."
In ancient Rome, a cohort was a battalion of soldiers marching under one banner — today it's the statisticians' favorite word for a group marching through life together.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cohort
Cohort comes from the Latin cohors (stem cohort-), meaning "enclosure, courtyard, company of soldiers," formed from com- ("together") plus hortus ("garden, enclosure") — the same root that gives English "horticulture." In Roman military organization a cohort was a unit of a legion, and by the early 17th century English had adopted the word both for this historical unit and, figuratively, for any band of followers. The statistical sense of a group sharing a demographic characteristic developed much later, gaining prominence in 20th-century social science.
How cohort is actually used
In academic and statistical contexts, "cohort" is neutral and precise, denoting any group defined by a shared attribute or time period. In casual usage, the plural "cohorts" often means companions or henchmen, sometimes carrying a slightly negative or comic connotation — style guides caution against using singular "cohort" simply to mean "friend," which many editors consider incorrect.
Easily confused with cohort
"Cohort" is the group itself, while "cohort effect" is a term in statistics and epidemiology describing how shared experiences of that group shape observed outcomes.