Slide film and transparencies are commonly called slide film I guess because most transparencies at least in 35mm used to be mounted for projection. They don't have to be mounted. It may be a colloquial presentation. But people use the word interchangeably especially for 35mm format.
https://www.walmart.com/ip/Fuji-Velvia-RVP-135-36-50ASA-Slide-Film/172289431
https://petapixel.com/2018/06/04/kodak-teases-first-test-photos-shot-on-new-ektachrome-film/
Yes you are correct many people make this mistake, it is still incorrect usage of the term.


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