In
philosophy, an accidental property is a
property that can be changed without changing the
essence of the thing it belongs too. For example, you can change hair styles, weight, and number of limbs and still be you.
This is in contrast to an essential property. You might not have been you if you had had a different genetic make-up or a different upbringing, so these may be essential properties. Of course, you can spend a lot of time arguing what are and are not essential properties.