ADHD is a neurodevelopmental condition where attention, activity levels and impulse control work differently from most people's, present since childhood and showing up across different parts of life.
ADHD stands for attention deficit hyperactivity disorder. That is the name the diagnostic manuals use. ICD-11 lists it as a neurodevelopmental disorder, and DSM-5-TR describes three presentations: mainly inattentive, mainly hyperactive-impulsive and combined.
In adults it rarely looks like the restless boy in the textbook. It looks like attention that goes to whatever is most interesting rather than most urgent. Restlessness that shows up as constant thinking more than as movement. Starting is hard. Stopping, once you're absorbed, can be just as hard.
A diagnosis needs the traits to have been there since childhood and to show up in more than one setting, such as home and work. That is why an assessment asks about your history, not only how this month has been. A screener can tell you whether the question is worth asking. It can't answer it.