Acronym for 'Big-Arsed Map' in the original Australian, and presumably 'Big-Ass Map' in American.
Description of a map, usually painstakingly cobbled together from little bits heavily zoomed in self-constructing Mercator maps off this website, which depicts an area (such as North America or Europe) with a scale that seems only slightly smaller than the actual area in real life.
Some people, notably Thande have criticised BAMs for crashing their computers, and have thus been labelled 'BAMophobes'.
The timeline Decades of Darkness appears to be the first in which BAMs were used, depicting North America and designed by Scarecrow. Unlike the majority of maps, BAMs tend to be made for specific timelines rather than as random exercises in themselves*. Their size allows a substantial level of detail, including towns, smaller provinces and divisions, etc.
*While this is typically true there are several blank BAMS available for anyone to use.