In a gamma or X-ray spectrum, the peak due to the photoelectric effect in the detector and escape, from the sensitive part of the detector, of the X-ray photon emitted as a result of the photoelectric effect.
Source:
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1541 (https://doi.org/10.1351/pac198254081533)