Agency Platform
Also known as: agency website platform, platform for agencies
The software a web shop builds and maintains client sites on, plus whatever it gives them for running the business around those sites.
An agency platform is the thing you build on when the website is for somebody else. It is a wider idea than a website builder, because a shop needs more than a way to make one page look good.
In practice a platform has to answer four questions. How does a new client site get built? How does it get maintained? What do you show the client each month? And what happens to all of it if you stop paying?
Those four questions are why this directory exists. Most marketing copy answers the first one well and goes quiet on the fourth.
The phrase gets used for two different products. Some agency platforms build websites. Others run the sales and reporting around websites somebody else built. Both are sold with the same two words, and buying the wrong one is a common and expensive mistake.