ADD, or attention deficit disorder, is often used to describe ADD without hyperactivity. According to the diagnostic criteria, the correct term for this would be "attention deficit hyperactivity disorder, predominately inattention type" which is rather long and wordy, so ADD is easier to say.
For more information:
ADD or ADHD: Which is Correct?
Differentiating ADD and ADHD: Diagnosis, Symptoms and Treatment
ADD vs. ADHD: Is One Better (or Worse) than the Other?

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