All of the hardware, software, networks, facilities, etc., that are required to develop, test, deliver, monitor, control, or support IT services. The term includes all of the information technology but not the people, processes, and documentation.
Infrastructure is the foundation. If the foundation is shaky, the house falls.
Infrastructure is no longer hardware; Infrastructure is Code. We don't rack servers; we write Terraform. This shift means infrastructure must be treated with the same discipline as software development: version control, code review, and automated testing apply to your network config just as much as your application logic.