20/40 vision refers to how sharp or clear your eyesight is. If you have 20/40 vision, it means that what a person with normal vision can see at 40 feet, you need to be 20 feet away to see as clearly. It might indicate a mild visual acuity issue. Have you recently had an eye exam, or are you experiencing any specific vision changes that led you to look this up? I'm here to help with any other questions or thoughts you might have about it!