There are a lot of people who have defined genius in a variety of ways over the centuries. Some gave better definitions than others. Here’s one I found as an example:
someone who has exceptional intellectual ability and originality
That definition doesn’t really capture the essence of genius though. I suspect that we each have our own ideas of what genius is, based on our own experiences. For that reason, the word becomes hard to define since we’ve all had different life experiences.
For me, genius is the natural ability to look at a problem and see the obvious that none have seen before. If you look at the best inventions and the most useful products, we tend to look at them and think, “Wow, now why didn’t I think of that?” Well I’ve found an example of genius. The simplicity of it is astounding. We’ve had running water and toothbrushes for many decades and only now has someone come up with the brush and rinse toothbrush.
Thanks to OhGizmo for pointing out the brush.