What is fugacity and why is it used instead of pressure?

Fugacity (f) is an adjusted pressure that accounts for non-ideal behavior - it represents the escaping tendency of a substance. For an ideal gas, fugacity equals pressure; for real gases and liquids, fugacity = phi * P (phi is fugacity coefficient). At phase equilibrium, fugacities are equal: f_vapor = f_liquid. Using fugacity instead of pressure allows the same equilibrium equations to work for both ideal and non-ideal systems. Fugacity coefficient phi captures deviation from ideal gas behavior.