a condition (mostly in boys) characterized by behavioral and learning disorders
"Many children diagnosed with ADHD exhibit attention deficits alongside hyperactive behaviors that can make learning difficult."
Initialism of attention deficit hyperactivity disorder.
"The teacher explained that ADHD is an initialism for attention deficit hyperactivity disorder, a condition affecting focus and impulse control in children."
In plain English: ADHD is a brain condition that makes it hard to focus, sit still, and control your impulses.
"Many students take medication to manage their ADHD symptoms during the school day."