I know that "opening number" is the more common term, but I was wondering if there exists a technical term known only by composers.

Specifically, I'm thinking about the kind of opening number that foreshadows all the music that will be performed later throughout the musical. For example, Jesus Christ Superstar: the opening number was basically a medley of the story's highlights.

Is there a special term for that kind of piece? If so, I imagine it would be an Italian expression.

"Overture" comes pretty close to doing this, though it isn't confined to musicals. Think Midsummer Night's Dream, or any Gilbert & Sullivan operetta, or Candide.

in a similar vein, prelude..

I would think that opening number more applied to the first stand-alone piece after the overture, but overture is the word Tito is looking for.

Yes, that seems to be the word. I'm a little disappointed in myself that I didn't think of that sooner. I thought there would be a specific, technical expression for the foreshadowing aspect.

" most notably in Gluck's, the overture began to foreshadow what was to come in the work's tunes"
http://encyclopedia2.thefreedictionary.com/Overture

Thanks everybody!