Inattention is a lack of attention: the mind is elsewhere, so you do not notice or do not stay with what you should. Teachers use it when a student drifts off. You can also have inattention to detail — missing small things — or inattention to a person, which can feel like you do not care. It is a slightly formal word, but not rare. Though actually it names a gap, not a plan, and not always a disease. Doctors may talk about inattention as one part of ADHD, but in ordinary English it just means “not paying attention.” It is also not the same as ignoring someone on purpose. Ignoring can be a choice. Inattention is often the mind wandering, even when the person did not mean to be unkind.