Mechanical aptitude was one's ability or aptitude to work with technology and machines.
Both Jake Sisko and Miles O'Brien placed in the lower third of his age group in regards to mechanical aptitude. When Benjamin Sisko notified O'Brien of Jake's limitations as the boy was about to work with him in 2370, O'Brien mentioned that he too was once considered to be inept mechanically, much to Captain Sisko's disbelief. O'Brien then added that he discovered talents he didn't know he possessed when stationed at the Cardassian front. (DS9: "Paradise")