When the concept of tzniut is discussed it is defined as 'modesty' and used overwhelmingly in regard to sexuality. However we know there are other uses for the term such as from the gemarah (Megillah 13b) which discusses the merit accrued to Rachel, Shaul and Esther because of their tzniut; none of which involve modesty in relation to clothing or sexuality. Similarly there is a stress on tzniut in the bathroom (brachos 62a/b) even when no one else is around. 

Is there a source for a **conceptual** definition of tzniut that would encompass all the various uses for the term?