Question

What is ionisation enthalpy?

Answer 1

The energy required to remove the completely outermost electron from gaseous atom in its ground state.this is called ionisation enthalpy.

Answer 2

ionization enthalpy of an element is defined as  the amount of energy required to remove "the most loosely" electron from the outermostmost shell of it's isolated gaseous atom in the ground state(don't worry it simply means the "normal state")
it is maximum for non-metals especially halogens(Cl,F,Br etc.) and least for metals especially alkali metals (lithium,sodium,potassium etc..)