From Academic Kids
In telecommunication, an interchangeability is a condition which exists when two or more items possess such functional and physical characteristics as to be equivalent in performance and durability, and are capable of being exchanged one for the other without alteration of the items themselves, or of adjoining items, except for adjustment, and without selection for fit and performance.
Source: from Federal Standard 1037C and from the Department of Defense Dictionary of Military and Associated Terms
In computer science, interchangeability is an ability that an object can be replaced by another object without affecting code using the object. That chance usually requires two objects share an interface that is either same strictly or compatible in particular case. See encapsulation article for detail.
In mechanical engineering interchangeability refers to standardised components, such as nuts and bolts, which are made to particular standards enabling say a nut made by one maker to fit a bolt made by another. Pioneering engineers like Henry Maudslay, Richard Roberts and Joseph Whitworth spearheaded this principle.