What defines the energy level at which an electron can escape from the atom?

The energy level at which an electron can escape from the atom is defined as the ionisation level. This is the threshold energy required to overcome the attractive forces between the electron and the nucleus. When an electron gains sufficient energy to reach this level, it can break free from the atom, resulting in ionisation.

Understanding the ionisation level is crucial in fields like atomic physics and chemistry, as it determines the stability of an atom and its ability to participate in chemical reactions. The ionisation energy differs for various elements, reflecting their unique electronic configurations and the strength of the nuclear charge influencing the electrons.

The other concepts included in the question, such as root mean square (rms) voltage, capacitance, and terminal potential difference, relate to electrical phenomena but do not specifically define the energy required for electrons to escape from atoms. Therefore, the ionisation level is the most accurate choice in this context.