Donna Elesee

Donna Elesee Beckett was a scientist at Project Quantum Leap and the wife of Dr. Sam Beckett. Due to Sam's swiss cheese memory and the Top Secret classification of any information regarding her, very little is known of Dr. Elesee. She was briefly reunited with Sam during "The Leap Back".

In the original timeline Sam and Donna never married as she jilted Sam at the altar because of issues with her estranged father that caused her intimacy problems with men. However, Sam's third leap was into one of her college professors and he managed to help her patch things up with her dad, presumably changing history and allowing them to marry as planned (Star-Crossed).

It is also possible that while Sam was jilted at their first wedding but rescheduled and the second time Donna showed up and Sam's swiss cheese memory just forgot AbOUT the second wedding. Neither theory has been officially explained on camera so they can both be seen as conjecture. The latter would explain why Sam remembered that they were married when he regained his memory during "The Leap Back".