Simple liquids are said to be Newtonian when the applied shear stress is directly proportional to the shear rate. In other words, the viscosity (applied shear stress/shear rate) of a Newtonian fluid is constant and does not depend on shear stress or shear rate at constant temperature.

NLGI licenses certification marks for use on labels of products that satisfy performance criteria and pass laboratory evaluations managed by NLGI. In 1989, GC/LB Certification Marks were designed and standardized by NLGI and ASTM (D4950) to meet requirements for lubrication of automotive wheel bearings and chasses and then written into many original equipment manufacturer (OEM) specifications. Chassis lubricants are designated L, and wheel bearing lubricants are designated G. There are two performance classifications for chassis greases (LA and LB), and three for wheel bearing greases (GA, GB, and GC). The automotive industry is in general agreement that the highest performance classifications (LB and GC) are suitable for service relubrication of vehicles. In 2020, NLGI introduced the HPM (High Performance Multiuse) Certification Marks. In addition to the general HPM Certification Mark, there are additional specifications and Marks for lubricating greases with enhanced water resistance (WR), corrosion resistance (CR), load carrying ability (HL), and low temperature performance (LT).

A numerical scale for classifying the consistency of lubricating greases, based on the ASTM D217 worked penetration at 25°C (77°F). NLGI Grades are also referred to as NLGI Consistency Numbers or NLGI Numbers. In order of increasing consistency (hardness):

Some grease suppliers use descriptions such as NLGI Number 1.5, which indicates that the grease is between NLGI Numbers 1 and 2.

Some fluids and many plastic solids, including lubricating grease, exhibit non-Newtonian behavior. In other words, the viscosity (applied shear stress/ shear rate) is not constant; instead, it depends on shear stress and shear rate at a given temperature. Thus, non-Newtonian fluids are described by their apparent viscosity. Conventional types of viscometers with uncontrolled shear rates are not suitable for measuring the viscosity of Non-Newtonian materials.

Any of several specially treated naturally occurring or synthetic materials, excepting the metallic soaps, which can be either thermally or mechanically dispersed in liquid lubricants to form lubricating grease. Sometimes called Synthetic Thickener, Inorganic Thickener, or Organic Thickener. Examples include polyurea, calcium sulfonate, bentonite, and silica-thickened greases.