Boiling
Boiling is rapid vaporization throughout a liquid that occurs at the boiling point, when the vapor pressure of the liquid equals the pressure above it.
Definition
Boiling is the rapid phase change from liquid to gas that happens throughout a liquid, not just at its surface, forming rising bubbles of vapor once the liquid reaches its boiling point, the temperature at which its vapor pressure equals the surrounding pressure, letting bubbles nucleate and grow within the bulk liquid rather than only at the top. While a liquid boils its temperature stays constant, since the added heat goes entirely into converting liquid to gas rather than raising the temperature further, and because the boiling point depends on pressure, it follows the Clausius-Clapeyron equation, $\ln(P_2/P_1) = -\frac{\Delta H_{vap}}{R}(1/T_2 - 1/T_1)$, so lower pressure lowers the boiling point and dissolved solutes raise it, an effect called boiling-point elevation.
Example
Water in a pot boils at $100^\circ\text{C}$ at sea level, and the rising bubbles are water vapor forming right inside the liquid, not air; the same water boils at about $95^\circ\text{C}$ in Denver and near $70^\circ\text{C}$ atop Mount Everest because the air pressure is lower there, which is why pasta takes longer to cook at altitude. Using $\Delta H_{vap} = 40.7\text{ kJ/mol}$ for water, dropping the pressure to $0.70\text{ atm}$ predicts a boiling point near $90^\circ\text{C}$, while a pressure cooker at $2\text{ atm}$ boils water at about $120^\circ\text{C}$, cutting cooking times roughly in half.
Key Insight
Turning up the stove under already-boiling water does not make it hotter, it just makes it boil faster, so cooking at a truly higher temperature requires raising the pressure with something like a pressure cooker. Superheating happens when a very smooth container gives bubbles nowhere to nucleate, letting the liquid climb above its boiling point until it suddenly erupts, which is why boiling chips provide nucleation sites and why microwaved water can explode violently when a spoon is dropped in.