A rebuilt engine is a used engine that has been partially reworked. It should run like new, yet it isn’t failed or broken down completely.
A reconditioned motor is one that has failed and broken down and was rebuilt. However, reconditioning has no legal definition and thus, depending on supplier and manufacturer, terms can differ.
Finally, a remanufactured car engine is a unit that has been taken back to the manufacturer or a specialised workshop for a ground-up rework. Everything is cleaned and changed to work just as good as new, yet it takes longer than with any process mentioned above.
Remanufacturing, reconditioning and rebuilding are different terms and should never be confused.