Plain English Meaning
OpSpecs are the official FAA paperwork that tells a commercial operator exactly what kinds of flying they are allowed to do, and the rules they must follow while doing it. Each operator has its own set, tailored to their certificate.
Full Definition
A set of FAA-issued documents that authorize a specific commercial operator (such as an air carrier or charter company) to conduct certain types of operations, and that spell out the conditions, limitations, and procedures under which those operations must be conducted. OpSpecs are issued under the operator's air carrier or operating certificate and form a binding part of how that operator is permitted to fly.
Context Anchor
Seen in FAA handbooks, company manuals, and instrument procedure discussions when a certain procedure or type of operation is allowed only for operators specifically approved by the FAA.