Viscoplasticity is the reaction to stress of a material which behaves as if it were composed of a plastic solid and a viscous fluid in which the flow is dependent on certain parameters.
The parameters affecting flow are, amongst others, temperature, load and load application gradient.
The Bingham non-linear model is often used to characterise a viscoplastic substance.
This substance will have plasticity and will not flow until a stress threshold t is reached. This is what happens when a thermoplastic is injected, for example.