Distributed Antenna System (DAS) - DAS is a network of antennas designed to provide coverage (wireless signals) to an area with little or no otherwise coverage. This could be inside a building or a small defined geographic region.
Distributed Unit (DU) - DU is a 5G term to describe the distributed unit which controls the RLC, MAC and parts of the PHY layer of the gNB. The DU is controlled by the Centralized Unit (CU) and is typically in close proximity to the radio unit (RU).
Downlink - Signal direction from tower to device.