Diffusion is the technical term for the tendency of particles to travel from an area of high concentration to low concentration, in the absence of any other forces or barriers, due to the random motion of the particles. For example, a gas in a room, such as CO2, will spread out until it is relatively evenly concentrated throughout the entire room. Alternatively, a liquid will move until it is level in its container.
Diffusion across a semi-permeable membrane is known as Osmosis. A semi-permeable membrane will allow some, but not all particles to pass through it. A common example of osmosis in practice is a cell membrane, which allows water in, but may prevent more complex molecules from passing through.