A disability was a type of impairment of one's mind or body.
A common belief was that those who lost one ability compensated by increasing their other abilities. When Picard tried to discourage an non-empathic Deanna Troi from resigning her position by reminding her of this in 2367, she said it was only a belief probably created by normal people who felt uncomfortable around disabled ones. Picard started to tell her about a female teacher he had at the Academy who was wheelchair-bound since birth, but she dismissed it. (TNG: "The Loss")
In Jean-Luc Picard's opinion, Doctor Beverly Crusher was asking too much of Worf to expect him to go from wanting to die to accepting his disabled condition following a freak accident in 2368. (TNG: "Ethics")
In 2373, Julian Bashir told an injured, reckless Martok that he should accept the fact that he had a disability after the one-eyed Klingon foolishly turned off the holodeck safety protocols during a battle simulation. (DS9: "Soldiers of the Empire ")