Question

what is ionising energy

Answer 1

the amount of energy required to remove the outer most bound electron is called ionisation energy

Answer 2

Ionization energy is the energy which required to remove an electron from a gaseous atom or ion. 
For Example: Atoms and molecules have a measurement called the ionization energy, which means that the amount of energy required to remove one electron while that substance is in a gaseous state is the amount of energy it takes for all the atoms in one mole of a substance to each lose one electron.