Energy level

Topic 15.3 models an atom as a nucleus plus an electron, and only certain interaction energies of that system are allowed. Those allowed values are the energy levels. Essential knowledge 15.1.A.5 states the general version: energy and momentum have discrete, or quantized, values for bound systems.

Ground state and excited state are sub-types rather than synonyms, and the CED names both in passing. 15.3.A.5 calls the lowest energy level the ground state; 15.3.A.2.ii has an atom in an excited energy state emitting a photon to move spontaneously to a lower energy state.

By the usual convention, levels are written as negative numbers on an energy level diagram, with zero taken as the electron just free of the atom. Negative means bound, so energy must be supplied to climb to zero. That is exactly why 15.3.A.5 says an atom in the ground state requires the greatest amount of energy to have its electron removed.

Transitions are where levels earn their keep. Energy is absorbed or emitted only when it matches the difference between two levels (15.3.A.2), and each transition corresponds to a single frequency and therefore a single wavelength (15.3.A.3). Convert with $E = hf$ and $\lambda = c/f$, or in one step with the printed $hc = 1240 \, \text{eV} \cdot \text{nm}$.

Only single-electron atoms appear, by the Topic 15.3 boundary statement.