The net movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration
Water concentration
The amount of water as compared to the other molecules like sugars or salts that are dissolved in that water
Dissolving molecules
Solutes
Beakers with different water concentrations
Beaker with 1 solute particle
Beaker with 3 solute particles
The beaker with more solute particles has a lower water concentration
The key point is that it's not the volume of water but the proportion of water compared to solutes that determines the concentration
Osmosis in cells
Water particles diffuse from the outside of the cell (higher water concentration) down the concentration gradient into the cell (lower water concentration)
The difference in water concentration between the outside and inside of the cell drives the process of osmosis