Strain is how a body responds to an applied stress. Rate of strain would be how quickly the cushion bows as you sit down. Stress on lava is exerted by the force of the eruption, the force of gravity, or forces exerted during cooling. Rate of strain is how quickly the flow deforms. Rate of strain may be high on steep slopes and low on gentle slopes.
Viscosity is the resistance to flow.
Viscosity can be demonstrated by pouring different liquids down a board. The viscosity lava is greatly influenced by composition and temperature.