For some people, it's a fictional technology that is detailed down to the very nuts and bolts. For others, a fictional culture that has all its elements seamlessly knit together to create a complex tapestry. A history that deftly tells the story of a person, nation, or planet, or an otherworldly species that feels real enough that it could exist, if only in another world.

What is it for you? What examples in fiction stood out for you? Why did they do; what about them spoke to you so strongly? It could be widely-known published fiction, or some niche project you ran into on the internet once.