Define or tell me the definition of venous tone?

A bit complicated: Venous tone is a property of the venous system, reflecting the venous resistance and pressure inside the veins. An increased venous tone caused by venoconstriction leads to higher venous resistance and venous pressure.

Tension of veins: Veins (and arteries) are like pipes that carry blood throughout the body. However, unlike the pipes that carry your water, they are reactive, and can dilate or constrict in response to various chemicals, both internal and external. Venous tone refers to the general or baseline state of constriction or dilation of your veins. Very constricted veins have high venous tone.