Enthalpy
Definition
A measure of a system's total heat content at constant pressure. Chemists track its change to say whether a reaction gives off or absorbs heat.
Symbol: · Unit: kJ/mol
Technical definition
Enthalpy (H) is a state function defined as internal energy plus pressure-volume work (H = U + PV). Only changes are measurable: at constant pressure the enthalpy change equals the heat flow, and standard enthalpies of formation enable Hess's law calculations.
Example
Burning methane has an enthalpy change of -890 kJ/mol. The negative sign means 890 kJ of heat leaves the system per mole burned.
Common confusion
You can never measure absolute enthalpy, only changes. And the enthalpy change tells you about heat flow, not whether a reaction happens spontaneously: that is Gibbs energy's job.