A robot is a machine that can make decisions on its own without manual input, right? And androids are robots that appear mostly human.
So should android -> robot, and robot -> machine?

(I did a search and found a topic on a similar theme, but it was 2 years old and didn't seem to reach a conclusion; pardon if I should have bumped that instead)

For Danbooru purposes, android is for things like post #923087, robot is for things like post #970300, and machine is for things like post #858068, which are all quite different visually so these implications would be a bad idea.

Remember, tags aren't so much about defining things as they are about being able to find images of things. A user searching for pictures of Asimo-style robots probably wouldn't want pictures of Kos-mos-style robots showing up in his search.

Good to know, I was always unsure how to tag when uploading megaman stuff.