润滑脂乃是在基础油加入增稠剂与润滑添加剂制成的半固态机械零件润滑剂。润滑脂俗称黄油、牛油，因为润滑脂是半固态的油腻物质且多半呈深浅不一的黄色~乳白色，与一般人习见的奶油、牛油很像，因而得名。

润滑脂的「脂」字和动物性油脂、化学的脂肪酸脂肪族没有关系。动物性脂肪主要是三酸甘油脂、甘油与脂肪酸所组成，只是熔点较高的有机化合物，加热即可融化、冷却即可固化。 但润滑脂是以增稠剂（thickener）吸附液态基础油而成，加热到滴点(en:Dropping point)以上会融化，冷却后无法恢复半固态，因为滴点是增稠剂的永久失效温度。

Grease is a semisolid lubricant. Grease generally consists of a soap emulsified with mineral or vegetable oil. The characteristic feature of greases is that they possess a high initial viscosity, which upon the application of shear, drops to give the effect of an oil-lubricated bearing of approximately the same viscosity as the base oil used in the grease. This change in viscosity is called shear thinning. Grease is sometimes used to describe lubricating materials that are simply soft solids or high viscosity liquids, but these materials do not exhibit the shear-thinning properties characteristic of the classical grease. For example, petroleum jellies such as Vaseline are not generally classified as greases.