Any tools / means that radiate out the information by making it visible and transparent to other team members, stakeholders and relevant parties. Usually Big Visible Charts, Large Displays, WhiteBoards, etc. may be used to make information visible.
Information such as: Product Backlog, Sprint Backlog, Defects Open, Impediments, Status of Build etc. may be made available through information radiators so that the team members can regularly inspect and adapt as needed.