Fat tree is a topology for data center networks. Compared to a normal tree topology, the links in the upper part of the "fat tree" have higher bandwidth and thus are fatter than the bottom part of the tree. From a theoretical standpoint, however, there is a proof that a fat-tree of a given size is nearly the best routing network of that size. The paper "Fat-Trees: Universal Networks for Hardware-Efficient Supercomputing" might be helpful for more details about fat tree.
This "blade" term is new for me. Quote from wikipedia : A blade server is a stripped down server computer with a modular design optimized to minimize the use of physical space and energy. Whereas a standard rack-mount server can function with (at least) a power cord and network cable, blade servers have many components removed to save space, minimize power consumption and other considerations, while still having all the functional components to be considered a computer.
so i guess in short, it is computer with efficient physical space and energy usage usually used for server. cmiiw
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