Osmosis
Definition
Water moving across a semipermeable membrane from the less-concentrated side to the more-concentrated side: how plant roots absorb water and why your fingers wrinkle in the bath.
Technical definition
Osmosis is the net flow of solvent through a semipermeable membrane toward the side of higher solute concentration, driven by the chemical potential difference. The pressure required to stop it is the osmotic pressure.
Example
Red blood cells placed in pure water swell and burst as water rushes in, which is why IV fluids are saline, matched to blood's concentration.
Common confusion
In osmosis the SOLVENT moves, not the solute. The membrane blocks solute, so water shifts to equalize concentrations: diffusion of solute is a different process.