One definition of being over-corrected is that your corrective lenses allow you to see better than an average non-myopic person i.e. you have better than 20/20 vision. It is not clearly defined at which ambient light level this is to be considered over-correction. However, it should be considered that opticians often prescribe lenses that correct you to at least 20/20 night vision on a 6m Snellen chart, therefore you may see better than 20/20 in bright sunlight without it being considered over-correction. If you can see better than 20/20 on a 6m Snellen chart at night with low levels of ambient light, it is probable that you are over-corrected.