_____acts as a lubricant in a synovial joint.

Answer

**Step-by-Step Solution:**

1. **Understand Synovial Joints**: Begin by defining what a synovial joint is. Synovial joints are the type of joints that allow for a wide range of motion between the bones, such as the joints in the limbs (e.g., shoulder, hip, elbow, and knee).

2. **Identify the Lubricant**: The question asks for the substance that acts as a lubricant in these joints.

3. **Recognize the Role of Synovial Fluid**: The lubricant in a synovial joint is synovial fluid. This fluid is a viscous liquid found in the synovial cavity of the joint.

4. **Function of Synovial Fluid**: Explain the function of synovial fluid, which is to reduce friction between the articular cartilages of the synovial joint during movement. This is crucial for smooth and pain-free motion in the joints.

5. **Conclude the Answer**: Therefore, the answer to the question is that synovial fluid acts as a lubricant in a synovial joint.

**Final Answer**: Synovial fluid acts as a lubricant in a synovial joint.

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