What is lucency on xrays?

Dark spot/ region. A lucency is a dark spot or region on an x-ray. It's significance depends on where on the x-ray the lucency is seen , e.g. Bone, lung, etc...
Describes a finding. In terms of x-ray exams, the term "lucency" is used to describe areas that are more dark (less opaque/white). "lucency" is a word used to report a finding, not a diagnosis, and it can have different implications depending on what anatomical region the "lucency" was identified in.