What are your weaknesses?
The candidate's response to this question provides the interviewer with an insight into the shortfalls of that individual.
Any employer will tell you that no employee is perfect. We all have our faults. We are all weak in some areas. Weaknesses are areas where you come up short. It would help if you improved on that area, so it's typically considered a weakness.
Some individuals genuinely don't have any weaknesses that they display at work. If that truly is the case, then perhaps mention something that you would like to get better at that may not be a weakness but not your greatest strength.