Define ionisation potential.

Explanation

**Step-by-Step Solution:**

1. **Understanding Atoms and Electrons**: An atom consists of a nucleus made up of protons and neutrons, with electrons revolving around the nucleus in specific energy levels or shells.

2. **Identifying the Outer Shell**: The outermost shell of an atom contains the valence electrons, which are the electrons that can be influenced by external energy.

3. **Energy Addition**: When energy is supplied to an atom, it can cause the electrons in the outer shell to gain enough energy to overcome the attractive force of the nucleus.

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