Fluid That Lubricates Joints
Puzzle 10 of Yoga Hard Pack
Question is: Fluid That Lubricates Joints
and the answer is: SYNOVIAL
Word Meaning:
SYNOVIAL — (anatomy) Relating to or denoting a type of joint which is surrounded by a thick flexible membrane forming a sac into which is secreted a viscous fluid that lubricates the joint.
Definitions from Wiktionary.