An imperfection is a detail that makes something (or someone) less than perfect. A tiny crack in your otherwise flawless Ming vase might be its only imperfection.
In addition to a blemish or defect, imperfection can also mean "the general state of not being perfect." For example, you could say, "Yeah, my dog is funny-looking, but her imperfection is part of what I love about her." The Latin imperfectus is at the root of imperfection, meaning "unfinished, incomplete, or immature," a combination of im-, "not," and perfectus, "finished or complete."