There is no single definition of"reverse proxy" and it seems that everybody has a slightly different opinion on the exact meaning of the term. The HTTP protocol definition RFC 2616 does not mention the term "reverse proxy" at all. In the terminology of RFC 2616 the SAP Web Dispatcher is a gateway:
A server which acts as an intermediary for some other server.
Unlike a proxy, a gateway receives requests as if it were the
origin server for the requested resource; the requesting client
may not be aware that it is communicating with a gateway.
The term "reverse proxy" is just a common synonym for such a "gateway", and since it's more common, let's stick with it. In addition to the RFC 2616 definition, reverse proxies can have many features. Most common are the following:
- SSL termination and off-loading
- Load balancing
- Decoupling of servers from slow front-end networks
- Virtual hosting of multiple backend systems behind a single IP address
- Request filtering based on simple rules, like URL paths
- Sophisticated application firewall features against web-based attacks
- Authentication and single-sign-on (SSO)
- Authorization of resource access per user
SAP Web Dispatcher provides all of these features except the last three. In terms of Security SAP Web Dispatcher has features like Mitigating Denial of Service (DoS) Attacks and although SAP Web Dispatcher does not have build in SSO feature it supports SSO see for example Single Sign-On for SAP Fiori Launchpad-Integrated Cloud Services. SAP Web Dispatcher has other security features described in Security Information for SAP Web Dispatcher
So the answer is: SAP Web Dispatcher is a reverse proxy. However, there are other reverse proxies that offer features that SAP Web Dispatcher does not have.