If you have seen the abbreviation BSE in a scientific article or image, it probably stands for Backscattered Electron.
Backscattered Electron (BSE) is a type of electron that is scattered by an electron beam. Or, more specifically, BSE is a mode in the scanning electron microscope (SEM) that uses these backscattered electrons.
Backscattered electrons (BSEs) come from the elastic scattering between the SEM electron gun and the atoms in the sample. Detecting BSEs provides information about the sample atomic weight. Heavy elements will appear brighter because they are more likely to generate BSEs.
Backscatter electrons (BSEs) are one type of imaging mode in the SEM. Secondary electrons (SE) are another mode.
This article is a definition. If you want to learn more about SEM, including SE and BSE techniques, check out this full article!