IPA: //ˈdaɪəpəˌzɪtɪv//
KK: /ˈdaɪəˌpɒzɪtɪv/
A clear image on transparent material, such as glass or film, used for projecting pictures with a projector.
The teacher used a diapositive to show the students images of the solar system.
Diapositive is formed from "dia-" (meaning through or across) and "positus" (from the verb "ponere", meaning to place). The word refers to something that is placed across or through, often used in the context of slides in photography or presentations.
Think of something being 'placed across' ('dia-') — that's why a diapositive refers to a slide that shows an image.