We sometimes call them "specs," but a specification has no relationship to eyeglasses! Instead, it is an explicit detail of a design, presenting the exact way something specific must be constructed. For example, blueprints contain the specifications for a building.
In the 17th century, the word specification emerged with the meaning of giving something a specific quality. The technical meaning, applied to building and engineering plans, developed in the late 18th century. The meaning of the word is precise, referring to specific plans, although just because it's a specification doesn't mean it is accurate. Science-fiction writer Robert Heinlein once had his character Lazarus Long make the comment, "An elephant is a mouse built to government specifications."