A hesitation is a pause. If your friend asks you, "Do you like my new haircut?" you'd better make sure there's not a second's hesitation before you answer, "Yes, of course!"
Hesitation happens when you feel uncertainty or doubt. It can be a pause in speech, a faltering moment before you act, or a silent second of indecision. If your friend's dog is growling and staring you down, but she tells you he won't bite, that feeling that something not feeling right that stops you from petting him is hesitation. The Latin root of hesitation is haesitationem, which means irresolution or uncertainty.