Webby's got it: That which is "necessary to be delivered", that's the one. It's a noun now.
Here in this grim and sordid butt-end of history, the term "deliverable" is used (always in the plural, as I recall) to refer to that product which is owned under a contract or agreement. When somebody agrees to pay you n dollars to write a program that does x, y, and z, the program is the "deliverable".
It's one of those words that sales and marketing people like to use. It sounds so businessy and professional. Look at us, we're running a company. How 'bout that?
Idiot "A" might contract with Idiot "B" to buy nineteen dozen of something A's already got, and that they're both familiar with: Plates or some such. That's easy. There's very little fear and loathing potential in that: It is what it is and you both know what it is. You can pick a few up and throw them at each other, just to be sure. It only gets weird when the thing to be delivered does not yet exist.
When you're doing any kind of development, it's a good idea to have the deliverables very clearly and narrowly defined at the outset. Otherwise, you'll be getting a phone call in a week: "Gee, we need it to do this, too." (Well, you'll get that phone call anyway, but if you've worked it right you can tell 'em to forget it.)
And another week after that: "Did we tell you it has to do such and such?"
Meanwhile, you've started designing the thing. Worse, you promised it by a given date, but now there's more work to do. This can wreck your schedule, and customers don't often admit that a slipped schedule is their fault. It may be too late in the game to make the change at all.
Then again, you might deliver the ill-defined deliverables and find yourself met with stunned silence: "That is not what I meant at all." It's too late now. Hope you weren't relying on that check, there, pal...
So you go through an agonizing and fantastically boring process of defining every last feature of the product up front. The resulting document is called a "spec", short for "specification". It specifies things, and for the reasons above should be very specific.