Wide Area Network (WAN) is inter-connected Local Area Networks (LAN), which can be WiFi or Ethernet networks. If you have multiple sites connected to each other over the internet, or remote users accessing your company resources, then you have a WAN.
Software Defined Networking (SDN) is a modern way of controlling network hardware by software running in the cloud. SDN not only provides savings in hardware, but also allows integration to other systems to have many powerful features such as faster setup, improved handling of network failures, better cybersecurity, etc.
Software Defined Wide Area Network (SD-WAN) uses the SDN approach for controlling WAN hardware, usually called "WAN router", or simply “router”. Using cloud management, SD-WAN easily and securely inter-connects routers over the internet, provides multiple ISP connections for redundancy, allows remote users to connect securely, etc.
Secure Access Service Edge (SASE) is a new network and cybersecurity architecture combining SD-WAN, ZTNA, and threat prevention. It manages access to on-prem or cloud applications based on users’ roles, locations and schedules. It also provides multi-layer cybersecurity with IP filtering, DNS filtering, CountryBlock, AdBlock, and Intrusion Prevention System (IPS). In addition, it monitors networks and users, discovers new devices, and reports significant events as well as suspicious activities in real-time.