Eugene Bingham, a colloid chemist, first coined the term
“Rheology.” He also showed that for many real fluids a
critical level of stress must be attained in order to initiate
flow. Below this critical stress, τy, the material behaves as a
solid, absorbing the stress energy without flowing. Once the
threshold of critical stress has been reached, the material
yields to flow, hence the term, yield stress. The yield stress is the
reason, why you need to shake or tap a bottle to make the
ketchup flow. Materials which exhibit Newtonian flow beyond
the yield bear the name Bingham Fluids.