Movement of water across the membrane by diffusion is:

Answer

### Step-by-Step Solution:

1. **Understanding the Plasma Membrane**: The plasma membrane is a selectively permeable barrier that controls the movement of substances in and out of the cell.

2. **Defining Diffusion**: Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration.

3. **Identifying Passive Transport**: The movement of substances across the plasma membrane can occur through passive transport, which does not require energy. This type of transport occurs along the concentration gradient.

4. **Applying the Concept to Water**: When considering the movement of water across the plasma membrane, it also follows the principle of diffusion, moving from areas of higher water concentration to areas of lower water concentration.

5. **Introducing Osmosis**: The specific term used for the movement of water across a selectively permeable membrane by diffusion is called osmosis.

6. **Conclusion**: Therefore, the movement of water across the membrane by diffusion is referred to as osmosis.

### Final Answer:
The movement of water across the membrane by diffusion is called **osmosis**.

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