In Electron Spin Resonance (ESR) spectroscopy, an absorption spectrum is considered to be a single absorption spectrum when it displays a solitary resonance peak that corresponds to the transition between the magnetic energy levels of unpaired electrons. This condition is typically met when the sample has a single paramagnetic species with an isolated electronic environment, meaning there are no significant interactions or couplings with other unpaired electrons or nearby nuclei. The magnetic field should be uniform, and the sample should be placed in a non-perturbing matrix to ensure that the resonance condition is only influenced by the inherent properties of the paramagnetic center. Under these conditions, the ESR spectrum will exhibit a single, well-defined absorption peak, indicating a singular resonance event.