Coworker simply means someone who works at the same place as you, whatever they do. Colleague carries a hint of shared profession or comparable standing — it is what a doctor calls another doctor, at any hospital. In practice the two overlap heavily, and colleague is the more formal of the pair.
The core difference
- coworker — anyone at the same workplace. Neutral about role, rank or field.
- colleague — someone in the same profession or of comparable standing, often but not always at the same organisation.
The part people miss
A colleague need not work with you at all. Academics, doctors and lawyers routinely call people at other institutions colleagues, because the word tracks the profession rather than the payroll. Coworker cannot do that — it is tied to the workplace.
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Colleague is the more formal and is usual in British English for both senses. Coworker is more common in American English and in casual use. Neither is wrong; the choice mostly signals tone.
Spelling
Coworker, co-worker and (rarely) coöperative-style forms all appear. British style tends to hyphenate; American style increasingly closes it up. Pick one and be consistent.
Frequently asked questions
Can someone be a colleague if we work at different companies?
Yes. Colleague can track a shared profession rather than a shared employer, which is why doctors or academics use it across institutions.
Is coworker or co-worker correct?
Both are accepted. British style tends to hyphenate; American style increasingly closes it to coworker. Consistency matters more than the choice.
Which is more formal?
Colleague. It is the usual word in formal and British English, while coworker is more casual and more common in American usage.