Explanation:

“Cryo”, short for cryogenic refers to very low temperatures. Though the actual temperature is not well defined, it is below minus 150°C.
In the context of electron microscopy, it refers to the fact that the object to be imaged is frozen to such low temperatures to facilitate being studied under the beam of the electron microscope.
This method is so effective that even in recent times, it has been used to image the elusive Zika virus: When researchers began to suspect that the Zika virus was causing the epidemic of brain-damaged newborns in Brazil, they turned to cryo-EM to visualise the virus.
Over a few months, three dimensional (3D) images of the virus at atomic resolution were generated and researchers could start searching for potential targets for pharmaceuticals.