I think it's so difficult to define because our own thoughts may not be truly original. I'd say plagiarism could be defined in proportion to the creative effort involved. If your work is composed mostly by your own ideas(even if you learned them from somewhere else) without trying to copy, its good enough.
Also I have mixed feelings about steemcleaners and cheetah. I've seen them censoring people by mistake. Or even going further and censoring entire blogs for being called nazis.
RE: Plagiarising Another's World - What is Originality & the Philosophy of Worlds