Final answer:
Everett Rogers outlined that the key element differentiating diffusion from other communication types is the adoption of innovations over time, potentially reaching complete societal saturation.
Step-by-step explanation:
According to Everett Rogers, the element that differentiates diffusion from other types of communications research is the concept of innovations being adopted over time within a society, resulting in varying degrees of market share, and potentially reaching complete saturation. This process outlines how different groups and individuals adopt new technologies, ideas, or practices, influencing social patterns and cultural norms. Rogers' model is significant in understanding the progression from early adopters to the majority and laggards in the context of technological, business, and cultural changes.