Requisite means required or necessary for something: "the requisite experience." Prerequisite means required beforehand — something that must be in place before you can begin: "statistics is a prerequisite for the course." The pre- adds ordering: every prerequisite is requisite, but not every requisite has to come first.
The core difference
- requisite — necessary for something, without a claim about when: She lacked the requisite qualifications.
- prerequisite — necessary before something can start: A first degree is a prerequisite for the doctorate.
Grammar worth noting
Requisite is chiefly an adjective, though it appears as a noun in formal writing (the requisites of a valid contract). Prerequisite is chiefly a noun and is common in education, where it names a course you must pass first.
Where people go wrong
Prerequisite is often reached for simply because it sounds weightier. If the sequencing is not part of your point, requisite — or plain required — is the better word. Saying a skill is a prerequisite claims that it must be held in advance, which is a stronger statement than you may intend.
Origins
Both are built on Latin requirere, to seek or need. Prae- supplies "before", and that prefix is the entire difference.
Frequently asked questions
Is every prerequisite a requisite?
Yes. A prerequisite is a requisite with timing attached — it must be satisfied beforehand.
Can I just write "required"?
Usually, and often you should. Required is plainer than requisite and carries the same meaning where ordering is not the point.
Is requisite a noun or an adjective?
Mainly an adjective, but it works as a noun in formal writing — for example, the requisites of a valid contract.