Osmotic pressure
Osmotic pressure (π) is the pressure needed to stop the flow of solvent through a semipermeable membrane from a dilute to a concentrated solution. It's a colligative property used heavily in biology and medicine.
π = iMRT
Where i = van't Hoff factor, M = molarity (mol/L), R = 0.08206 L·atm/mol·K, T = temperature (K).
Biological relevance
Blood plasma has an osmotic pressure of about 7.6 atm at 37°C. IV solutions must be isotonic (same osmolarity as blood, ~0.3 M) to prevent cell lysis or crenation. Reverse osmosis uses applied pressure exceeding osmotic pressure to purify water.