The field of Science communication (SciCom) is exactly what it says on the tin, work that seeks to communicate science. The focus is primarily on communicating science to non scientist audiences, but it also functionally seeks to effectively communicate science to other scientists.
Informal Science Education (ISE), describes the processes that lead to learning, specifically focusing on science or STEM subjects, in environments that fall outside the curricula and structure of schools (formal education).
What becomes clear, as you examine these two fields, is that they share a rather significant overlap. The National Science Foundation considers Science Communication to be one of the “sectors” of informal STEM learning (another entry will be needed to discuss how learning and education are used often interchangeably in a way that is nothing if not unhelpful). Within the framework of the relationships between ISE and SciComm, SciComm essentially describes literal communication (through literature, video, etc.) of science, which is a small slice of ISE, but in a more broad application the two are in many ways different labels for the same central concept.
There is some call within the academic literature for a “convergence” of the two fields into one, which I think deserves some traction. While my major reason for liking this idea is because of the significant semantic overlap, there is also some suggestion that because the two fields are separate avenues of research, much can be gained in terms of consilience (the combination of multiple pieces of evidence to generate stronger support than the sum of their parts). I find that I say ISE and SciComm interchangeably, mostly depending on the syntax of the sentence they’re nestled in. When it comes to terminology, this is less than adequate.