Exact-constraint design is a mechanical design philosophy by which mating parts are joined in such a way that they satisfy only the minimum number of spatial holding constraints to constrain motion to only the intended direction. The idea is that an object in 3D space has 6 degrees of freedom (3 translation and 3 rotation). By adding fasteners to attach this object to another, we cut down on these degrees of freedom until the object only moves in limited manner (or not at all) that we've intended. The notion of "exact" in exact-constraint design comes from the idea that adding additional fasteners in some ways will limit degrees of freedom that are already constrained. Doing so is redundant, resulting in a part set into a configuration that is "overconstrained."
This philosophy plays an important role in the design of instruments and machines with precision motion.