Yes, it is specifically defined. There are Institute standards for such things though the US and Europe often do not have the same base standards. Then there is ISO which is supposed to be a set of international standards but are not consistently followed very well by China for instance. Viscosity is a measure of resistance to shear, a kind of stress or force. High viscosity means higher resistance or generally a thicker oil. Low viscosity mean less resistance or a thinner substance that pours more easily.
Phae