* It has just dawned on me that readers under a certain age (whatever age that might be) don't know what a "slide" is. Well, pre-digital (ie film) cameras have three types of film available: normal "colour negative" that is then used to make prints, "monochrome" for black & white prints, and "colour reversal". The latter used special processing so that, once developed, rather than resulting in a colour-negative which had to be converted to positive via another colour-negative process (ie by making a print), the film itself becomes colour-positive (reversed from the negative original image) and can be viewed with light behind it - or projected onto a screen with a suitable optical system.
Unless you specified otherwise, your roll of 24 or 36 transparencies (35mm film) were returned from the processing lab chopped into individual frames, each sandwiched into a plastic or cardboard mounting to keep it flat and give you somewhere to hold it without putting finger marks on the film frame itself. The mount is (from memory) about 50mm square (possibly 50.8mm or 2 inches), with a rectangular aperture to reveal the photograph but hide the sprocket holes that run either side (top and bottom edge of the film if the photo is landscape, used so that toothed rollers inside the camera and processing equipment can grip the film without damaging the image area).
The later projectors have automated racks for the mounted transparencies, and projected each in turn at the press of a button on the projector itself, on a long control lead, or by wireless remote control (or by the "wireless" control of presenting the show from the front and telling your projectionist "next please"). The best projectors have auto-focus and lesser projectors have manual motorised control, or the ultimate in austerity is to have to focus the projection lens yourself. It is not "set and forget", because the film is not co-planar in the mount - it is bowed, and then when it heats up in the projector (because of the powerful lamp used to illuminate the slide and cast an image onto the screen - darkened room required) it "pops" - that is, the bow flicks from one side to the other - and the lens needs to be re-focussed.
Being square, the mount can be fed into the projector in 8 orientations - four of which compensate for which way up the camera was when taking the photo in the first place, and the other four are similar but back-to-front. This leads to many opportunities for embarrassment (and audience ridicule) if you have loaded the rack incorrectly, bearing in mind the transparency in the rack has to be upside down. Dropping the rack at the last minute and just stuffing the transparencies back at random gives an 87.5% chance that any particular transparency will be on its side, upside down, and/or back to front.
So why "slide"? The pre-automatic projectors were just a lamp, a lens, and a method for manually positioning a transparency into the optical plane of the lens. This comprised a carriage with two holders for the mounted transparencies, which was then manually slid across so that one holder moved into the optical plane while the other became accessible to change the transparency. Hence the individual transparencies, for want of a shorter word, became known as "slides".
In the early days of TV, many static-frame images (particularly those used on a repeated basis) were slides.