A layer seven firewall that sits in line at the boundary between the internet and an organization's network perimeter that allows security policy enforcement and can perform certain prevention and detection tasks.
CyberWire Glossary link: https://thecyberwire.com/glossary/secure-web-gateway
Audio reference link: ‌Vintage Computer Federation (2015). VCF East 9.1 - Ches’ Computer Security Adventures - Bill Cheswick. YouTube. Available at: https://www.youtube.com/watch?v=trR1cuBtcPs.