When you don't pay full attention to something or someone, that's inattention. Your inattention while you're driving can lead to mistakes like running a red light or bumping into the car in front of you.
If you're not focusing on something enough, you're guilty of inattention. Inattention can be dangerous when you're flying a plane, taking a math test, or babysitting an active toddler. When you add the "not" prefix in- to attention (which comes from the Latin attendere, "give heed to," or literally "stretch toward"), you get inattention.