Straight Punch

There are two types of straight punch. One is called the Cross and the other is called the Jab. The difference between these two is the side of the fist you're using. If you're using the back fist (right fist for the right handed) and twist your body to swing the punch to hit the target, it is the Cross. If you're using the lead fist, it will be called the Jab.

Note that the Jab is not so powerful because it doesn't require the twist of your body or hip to develop the force, so it is usually followed by another attack from the other arm or leg.