What is Spare Part?

Legal Definition
A spare part, spare, service part, repair part, or replacement part, is an interchangeable part that is kept in an inventory and used for the repair or replacement of failed units. Spare parts are an important feature of logistics engineering and supply chain management, often comprising dedicated spare parts management systems.

Capital spares are spare parts which, although acknowledged to have a long life or a small chance of failure, would cause a long shutdown of equipment because it would take a long time to get a replacement for them.

Spare parts are an outgrowth of the industrial development of interchangeable parts and mass production.
-- Wikipedia
Legal Definition
A component that can be replaced when it is needed to be because of malfunction or wear.