Incidental property

Overview
In substance theory, an incidental property is any property of an object which is not essential to the object's "substance", i.e. not an essential property.

For example: a chair may be blue, tall, wide or narrow, made of wood or plastic. These properties are not essential to the essential properties which make the object a chair, which might be specified (depending on your definition of "chair") as sittability, stability, and movability.

Usage
This idea is more traditionally referred to as an "accidental property" or just an "accident". This usage can be confusing or ambiguous to modern ears ("accident" carrying the connotation of damage or a mistake), so we are using the more normative and unambiguous "incidental". "Circumstantial" would also be a reasonable substitute, i.e. "attributable to circumstance rather than nature".

Reference

 * Wikipedia: Accidental property, Accident (may soon merge with previous article)