I am not quite sure if this question is fitting here, as I am looking for one word and it's about cars. I have searched the internet and found nothing. I'm not even sure if there's a word for it. The word I'm looking for is the word for the thingy below the car door. Not below the car, it's kinda where you put your foot when you enter. Or, in a movie, where the bad-ass character puts his enemy's head and closes the door repeatedly. It's kind of next to the car seat, but it isn't really in the car. Hopefully there are some car experts here, though even a car expert might not have taken the time to find out what this little section is called.
You may be thinking of a running board. They are not that common on modern cars, but they have a rich history. https://en.m.wikipedia.org/wiki/Running_board
When referring to the horizontal segment of the rocker panel, it's called a sill. In some cases this step-over area on top of the rocker panel has a flat (decorative) piece attached, which is called a sill plate.