How well something or someone fits a particular purpose — the quality of being right for the job, the place, the person, or the situation. It is the noun of 'suitable'. Because it is a noun, it names the whole question rather than answering it: you talk about 'the suitability of the site for a school', meaning whether that site is right, how right it is, the matter of its rightness. It nearly always comes with 'of X' and very often 'for Y' or 'as Y'. This is a formal word. You meet it in reports, job adverts, planning documents, school and hospital paperwork — 'we will assess your suitability for the role', 'a report on the suitability of the building'. In everyday speech people simply say 'whether it's suitable' or 'whether it's right for'. If a student writes suitability in a chatty email it will sound like a form letter. One useful thing about it: it is neutral and a little distant, which is exactly why officials like it. Saying 'we have doubts about his suitability' is a gentle, deniable way of saying 'we don't think he should get it'.