What's a quantum leap? a) something you learn about in physics class; b) a series now completing its third season on NBC; c) a boost in ratings that would greatly help ''b.'' We're sure Scott Bakula's scientist Sam Beckett, star of Leap, would agree with the first two definitions. As for the third, well, it seems NBC which has renewed the 64th-rated show for next season is experimenting with a new technique for increasing viewership: wishful embellishment.
At a mid-April press lunch, an NBC exec announced that Sam, who spends each episode occupying the body of a different character, would leap into the re- nowned private investigator and former ratings whiz Thomas Magnum (Tom Selleck) of Magnum, P.I. in this season's May 20 finale. Well, never mind. Now, the word from NBC spokesman Bob Meyer is: ''We're hoping. It's on hold. We don't have an answer.'' Just what constitutes an answer? Selleck's publicist says: ''There's no truth to it.'' A spokesman for Donald P. Bellisario, Leap's executive producer, is also denying the rumor. As for the same NBC exec's ) wacky comment that Sam will hurtle into The Silence of the Lambs' psycho-shrink Hannibal Lecter next season, NBC now sheepishly claims that quantum leap was only a woolly joke. As of now, Sam Beckett doesn't plan to have any friends for dinner.