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Slot refers to one of the armor or weapon slots in the paper doll, see in the Inventory window. Players refer to slots when talking about their gear, as in "I've got legendaries in half my slots now."

(BiS) is a common acronym meaning "Best in Slot." This refers to the item or items that are the best possible for a given item slot, when no further upgrades are likely, or even possible.

Inventory and paper doll with all slots empty.