Correct me if I'm wrong but I think science fiction often confuses invisibility with transparency.
Even if somehow we could make a human being transparent, this wouldn't make them invisible, any more than a clear glass bottle is invisible. Of course the refractive index of glass is a large factor.
At the same time, in SciFi, there is often an assumption that, given enough time and technology, every trope will be achievable sooner or later.
Is there any conceivable real science or technology that can even theoretically allow invisibility of a human being? If not, how about any reasonably constituted alien that visits Earth?
The subject must be a living adult human (or alien).
The invisibility must apply for 360° around the person so that they would be unseen in a crowd from any direction. A helicopter or drone might be able to detect them from directly above.
The subject can be detected by touch and other senses - just not by normal human vision.
In clean dry air, they cannot be seen. However it is acceptable for dust, rain, fog etc. to reveal them.
Invisible aliens must have a solid body with plausible parts that allow them to move and live. They cannot be made of a pure gas for instance.