Is an explosion merely an exothermic reaction?
The key parts to the definition of an explosion are "rapid" and "increase in volume".

Exothermic reactions do not necessarily meet these requirements. Exothermic reactions need only release energy to the surroundings by converting chemical potential energy into kinetic energy.

For example, the combustion of methane is very exothermic, with a standard enthalpy change of -890 kJ/mol.

$\ce{CH4(g) + 2O2(g)-&gt;CO2(g) +2H2O(g)} \space  \space \space \space \Delta_c H^o=-890 \text{ kJ/mol}$

However, at room temperature this reaction is very slow. Mixtures of oxygen and methane do not spontaneously ignite without a spark source. Also, there is no change in pressure with this reaction, since there are 3 moles of gas on both sides of the equation.

On the other hand, consider nitroglycerin. Yes, it's combustion is significantly exothermic. More importantly, the combustion happens rapidly, needs only the heat of friction or the energy of impact to ignite, and produces significantly more gas than it consumes:

$\ce{4C3H5O9}(l) \ce{+1O2(g)-&gt;12CO2(g) +10H2O(g) +6N2(g)} \space  \space \space \space \Delta_c H^o=-5660 \text{ kJ/mol}$

All thermodynamic data were mined from the NIST Chemistry Webbook.