What is martial art?

There are several arts to train, and if you look at all the possibilities on techniques from all the art. Then you will think that it’s not any limits at all, and after my opinion there is no limits,
Is there any similarities?
If you look at arts you will see it, but there are a lot  of similarities between totally different arts also. Here is som of them:
  • Positioning
  • Movement
  • Center Line
  • Center of gravity
  • Distance
  • …..
So if I want to focus on learning more about Martial Art, then I would focus on   items that is similar through all styles.