It's where it first came to prominence. The earliest D3D api was immediate mode (meaning, bare metal, talking directly via an api to the hardware). Retained mode is about retaining statefulness and some better abstractions to the underlying needs of the hardware (eg automatic texture resampling, etc). Considering it's an NVidia guy talking: he's been around for a while.

You probably won't be surprised that OpenGL's classic programming mode is also called immediate mode :)

  glBegin(GL_TRIANGLES);
    glVertex3f(...);
    glVertex3f(...);
    glVertex3f(...);
  glEnd();

Interesting, thanks! That would explain why I and probably many other readers, who have built plenty of GUIs but have not worked specifically with APIs like DirectX, would not be familiar with the term.

The terms are in common use since the 80s at least (no, Microsoft did not invent these terms with D3D). You can prove this in google scholar.

In fact for 3D rendering, retained mode was touted as a the new way forward, there was even a standard predating OpenGL called PHIGS, a retained mode system.. OpenGL was the worse is better (also PHIGS had many limitations basically limiting its use to CAD) backlash promoting its immediate mode in the early 90s.

Retained Mode Direct3D is the first and only place I've really heard that term used. After D3D stopped offering Retained Mode I haven't heard anyone use it except for a couple of articles about "immediate mode GUIs". Last time I saw one of those I had to look it up.