This is the noun: the state or habit of not giving enough attention. If inattentively is how one action goes, inattentiveness is the stretch of time, or the pattern, when the mind keeps slipping off. A school report might mention inattentiveness in class; a doctor might ask about it if someone cannot stay with a task. It is not the same as being unkind, and it is not the same as being slow. Someone can care a lot and still show inattentiveness when they are tired, hungry, or full of other thoughts. The word feels formal and a little heavy, as if you are naming a quality. Though actually, in everyday English people more often say inattention, which is shorter and usually means the same thing. Friends almost never say inattentiveness over lunch; they say you were not focusing, or your mind was somewhere else.

不專心；心不在焉（指這種狀態）。學校、診所較常說「注意力不集中」。