That would be a "deck of slides." Nowadays it's a keynote, open office, or that-other-thing file, but once upon a time it was a set of 35mm transparencies you would load into a carousel.
Stacked up, they looked a little like a deck of cards, thus the nickname "deck." You can probably find one (and someone old enough to remember when they were called decks) in a museum somewhere.
I'm old enough to have presented from transparencies on an overhead projector, though I've never had occasion to present using an actual slide carousel.
BTW, I'm not sure I understand why you think it's so important to present with a Powerpoint (or similar). I tend to think Powerpoints get overused, and I often like to present working just from notes (3x5 cards, since I'm bad at memorizing) or from a demo when appropriate.
What's the win of working with a deck, in your view?
- Provide graphics to illustrate your points (including code).
...and really that's about it. I keep my slides to one graphic or max five words, plus a little flow chart of progress through the presentation along the bottom. Depending on what I am talking about I might use as many as 2 slides a minute, but usually one every 2-3 minutes.
This is the same for monthly reporting (twenty graphs, with a page number on each) through to conference speaking (scripted and learned, lots of pictures, little navigation helper on each page).
Uh... could it be that the author copied this comment and pasted it into the article when he realized that the word "deck" isn't commonplace? If I see him, I will be sure to ask him which came first, the comment or the update to the article.
He says it's the most important thing it the world to have for a talk you give, but it's the first I've heard of it...