Atemi is the hit. Typically, strikes are done with hands, fists, feet, ellbows, and knees.
This reminds me of a poem that Sensei Malachi Lee (Insshinryu master) had at the entrance of his dojo. It went something like this:
I come to you with karate my empty hands. I have no weapons. However, if I am forced to defend myself, my loved ones, my home, my honor, then here are my weapons Karate my empty hands.
That was so cool to see every time I entered the dojo. It just put me in the right frame of mind.
Since those days, I've broaden my view of atemi to include: a sweep, a squeeze, a pinch, a bump, a push, a twist, a look, a kiai, and my favorite, ki extention.