Still working properly; not damaged, not weakened. The word is used about abilities and functions rather than about objects — you talk about unimpaired vision, unimpaired hearing, unimpaired judgment, unimpaired movement. It is formal, and it belongs to medicine, law, and careful reports. A doctor writes that a patient's memory is unimpaired; a court asks whether a driver's judgment was unimpaired at the time. Behind the word there is nearly always a reason to worry — an accident, an illness, alcohol, old age — and the word answers that worry with "no, it is still fine". That is worth noticing, because it explains why you would not say a new bicycle is unimpaired. Nothing has threatened it yet.