A discussion of cryo-EM terminology as the outreach and number of PDB entries expand.

Cryo electron microscopy (cryo-EM) has made great advances in the last decade, progressively increasing its impact in structural biology as a key method to address molecular structures and mechanisms of various macromolecular complexes. Single-particle cryo-EM will soon equal the number of yearly entries in the Protein Data Bank from structures determined by X-ray crystallography. This is largely thanks to improved cryo electron microscope instrumentation and advanced image-processing tools and structure-sorting methods. As the role of cryo-EM is expanding to an increasingly large community, including newcomers and scientists joining from related fields, it is timely to revisit some fundamental concepts and basics of single-particle cryo-EM and image processing as terminology has become less well defined and, in some cases, confusing. Here we summarize and define some typical terms important for understanding the underlying physical concepts. These include `cryo-EM', `cryo-ET', `3D reconstruction', `coarsening', `contour level' and others. We also discuss resolution estimation and map deposition.