This is the 6227th most frequent German word.
dias
"Dias" in German means "slide" (as in a photographic slide).
Here, 'dias' refers to singular 'slide' as a physical transparent picture typically used in projectors.
Ich habe ein Dias in einem Projektor verwendet.
I used a slide in a projector.
In this example, 'dias' is plural (slides) and refers to a collection of such items.
Meine Sammlung von Dias zeigt die schönsten Sehenswürdigkeiten.
My collection of slides shows the most beautiful landmarks.
This use demonstrates that 'dias' is a countable noun referring to a specific slide.