LBM is short for Little Brown Mushroom, a handy handle for a polyphyleticgroup of mushrooms that are difficult to distinguish from one another. However, due to the existence of this term, you can say, "Oh! That's an LBM." and you will have the twin satisfactions of being right and impressing people who don't know better.
I should point out that the term LBM is not limited to merely brownmushrooms; it also encompasses mushrooms greyish brown, grey, and other dull shades. Any feature, color or otherwise, that might make the mushroom more interesting (hairs, scales, deliquescence) tends to exclude it from this category. Nor is the "little" absolute: a large, dull-colored, uninteresting mushroom is merely a large LBM.
Little Brown Brown-spored mushrooms, have long been the core of the LBM category, but recent advances in our knowledge of the little white-spored wood-rotting fungi (featured in this website as SmallLignoTrich) are just as confusing and difficult to define.
Similar categories in other disciplines include the LBBs (Little Brown Birds) of birders, and LGRs (Little Gray Rocks) of geologists.