Software-defined radio is a field at the intersection of several technological domains including radio, digital signal processing, integrated circuit design, and software-based control systems, among others.
The SDR Forum (now known as the Wireless Innovation Forum) and the Institute of Electrical and Electronic Engineers (IEEE) P1900.1 group formulated a definition of software-defined radio:
Radio in which some or all of the physical layer functions are software defined
Traditional radio hardware is limited in its capabilities and can only be modified using physical intervention with the radio device. However, in software-defined radio, antennas, receivers, signal processing systems, and other components can be adjusted via a software control layer.
Hardware devices could include field programmable gate arrays (FPGA), digital signal processors (DSP), general purpose processors, systems on a chip (SoC), and other types of programmable processors. The use of these technologies makes it possible to add wireless capabilities and features to existing radio systems without the need for new hardware.

Via the Wireless Innovation Forum, an industry group focused on SDR and related technologies.
There are a number of software packages which interact with radio hardware as part of a software-defined radio system. GNU Radio is a popular open source platform for software-defined radio applications.
SDR companies
Timeline
People
Further reading
Cognitive Radio for Public Safety
Web
February 11, 2011
Software Defined Radio - an overview | ScienceDirect Topics
Web
Software defined radio (SDR) foundations, technology tradeoffs: A survey
Rahul Krishnan, R. Ganesh Babu, S. Kaviya, N. Pragadeesh Kumar, C. Rahul, S. Santhana Raman
Conference
September 2017
Documentaries, videos and podcasts
5 Cool Things You Can Do With An RTL SDR Receiver
October 26, 2017
Chapter 01 Software Defined Radio SDRs || Introduction and basics of SDR || Practical case studies.
October 29, 2019
How It's Made - Software Defined Radio - SDRPlay
November 9, 2020
The Beginner's Guide To Software Defined Radio RTL-SDR
April 5, 2018