Question

what is standard enthalpy of fusion & combustion?

Answer 1

Enthalpy is basically a thermodynamic quantity. It is net heat content of the body. 
Mathematically, the enthalpy of a system H is defined as
H = U + pVwhere,
H is the enthalpy of the system
U is the internal energy of the system
p is the pressure at the boundary of the system and its surrounding
V is the volume of the system.