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Well, as far as the inset map itself, I would agree with @cl3 answer of "Locator Map". Now, if you're talking about the red box in your image marking the extent of the related map, I don't know if its the academically officially correct term or not, but ArcGIS Desktop software calls it an "Extent Indicator".
A "locator map" is what I have most commonly heard it referred to as.
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