What is the process called in which fluid moves across a semipermeable membrane from an area of lower concentration to higher concentration?

The process described in the question, where fluid moves across a semipermeable membrane from an area of lower concentration to higher concentration, is known as osmosis. Osmosis specifically refers to the movement of water molecules, and it occurs when there is a difference in solute concentration on either side of a membrane. In this case, water will move towards the area of higher solute concentration in an attempt to equalize the concentrations on both sides of the membrane.

This process is crucial in various biological and physiological contexts, including maintaining fluid balance in the body. For instance, in hemodialysis, understanding osmosis is vital as it helps to manage the movement of water and solutes during treatment.

Other processes mentioned, such as diffusion, involve the movement of solutes from an area of higher concentration to lower concentration, which does not align with the fluid movement specified in the question. Ultrafiltration and filtration pertain to the movement of larger particles and fluids driven by pressure rather than concentration gradients, which further differentiates them from osmosis.