A term which denotes the validity of exchanging an identified physical item with another identified physical item. There are three types of equivalent:

• Interchangeable item, where an interchangeable item is equivalent in fit, form and function to the item for which it is exchanged. It can be used without selection of fit or performance, or without alteration to the item or adjoining items, except for adjustment.
• Replacement item, where a replacement item is equivalent in function but differs physically from the item for which it is exchanged. Installation of the replacement item may require additional drilling, reaming, cutting, etc. to the item or adjoining items.
• Substitute item, where a substitute item can only be used under specified conditions or in particular applications, without alteration to the item or adjoining items

See also FIT, FORM AND FUNCTION, and PHYSICAL ITEM.