Greases, pastes, and compounds are semi-solid lubricants used when liquid oils can't be effectively retained within an assembly, due to open structural designs, low operational speeds, intermittent motion, or sealing constraints. Rather than circulating like an oil, they stay in place, sealing out contamination while releasing lubrication as needed.
Grease is not simply thick oil. A thickener creates a structured three-dimensional matrix that holds base oil in suspension, responds to mechanical shear, and controls how oil is released and replenished at the contact zone. Pastes and compounds carry the same idea further with much higher solid content, providing high load capacity, anti-seize behavior, and resistance to squeeze-out, though this can also make them unsuitable for high-speed rolling bearings, fine centralized systems, or small clearances. Selection depends on operating speed, temperature, load, contamination exposure, and whether the application needs anti-wear protection, dielectric insulation, or assembly anti-seize behavior.