Refers to a substance used to reduce friction between surfaces, in mechanical, industrial, or metaphorical contexts.
機械の部品に潤滑剤をさす。
Apply lubricant to the machine parts.
潤滑剤 covers any lubricating agent, whether solid, liquid, or grease, while 潤滑油 refers specifically to liquid oil.
Compound of 潤滑 (jun-katsu, 'lubrication') + 剤 (zai, 'agent, substance').