I’ve started using this term as a combination of two other terms:
The Overton Window is the range of political opinions that are broadly acceptable. In normal-ish times, these opinions span the statements of mainstream politicians, who are trying to build broad (and overlapping) bases of support, but who maneuver within the range of acceptable opinions for strategic and philosophical reasons.
The Dunbar Number is the number of stable social relationships a person can maintain. Depending on the definitions and tests used, it ranges between about 100 and 250, with 150 being a rough rule of thumb.
My term “Dunbar Window” is then “the range of political opinions which are broadly acceptable in an individual’s social circle.
Dynamics and characterization of networks - including human social networks - is an area of ongoing research. Some models of how rumors spread distinguish between who has heard a meme and who is likely to share it. You can imagine a graph like the following, with each node representing a person and each line being a connection between them. A real community would have many more nodes, and many more connections. It would also have different types of connections, weights showing the strength of the relationships between them, and directionality showing that relationships are not necessarily symmetric.
Simplified network graph, to model people (nodes) and their relationships (edges). Colors may be used to describe different subgroups, which could be assigned in different ways.
Information, rumor, emotion, and political leanings all propagate along these edges: We tend to adopt the beliefs and opinions of those whom we trust and with whom we regularly communicate. We filter the information we receive before passing it on to others, choosing what to propagate and how to combine conflicting reports we receive from our contacts.
Since first-hand information is most reliable, long paths in this sort of diagram are less trustworthy: Many people will have had the opportunity to have misunderstood (or misrepresented) the information they pass along - much like a game of “telephone” in which there are occasionally incentives to lie.
Note also that some portions of the graph are only weakly connected to the rest: See, for example, the five red nodes at the top. These few nodes can only obtain information about the rest of the community through one other node. If the person represented by that node does not convey accurate information, they can effectively mislead the subgroup they lead, perhaps forming them into a cult beholden to the single gatekeeper who provides information about the outside world. Having multiple news sources and individuals with different assumptions and associations is therefore helpful in avoiding misinformation.
Understanding an in individual’s (or group’s) Dunbar window is essential to understanding their politics and their blind spots.