What is the best term for a UI showing large image or image gallery in a modal?
Two common use cases are e-commerce sites and photo galleries.
The first term that comes to mind is lightbox as the original JS library was called like that and the terms seems to have caught on in the JS / web ecosystem, e.g., "7 Best WordPress Lightbox Plugins" or "How to create a lightbox".
One possible reservation is that "lightbox" generally means "a piece of UI that is above the page, with the rest of the page dimmed". Even if the modal is very small, it is still a "lightbox UI".
I quite liked the term "theater" mentioned here – that makes me think about something taking almost the full screen, which is good. However, I've never seen this term used in any UI library / toolkit I've ever used, unlike e.g. "hero" (which is also "strange" but once you get used to it, it is clear and unique).