I'm pretty sure the acronym was for "Physically Based Ray Tracing". The original preface to The Book explicitly says that

> pbrt is based on the ray-tracing algorithm

I suspect it's moved to "Rendering" in common usage to avoid getting hung up on distinctions like ray tracing versus path tracing versus hybrid techniques.

And specifically a reference to a book, which describes an implementation: https://www.pbrt.org/

And is available to read online legally here:

http://www.pbr-book.org/3ed-2018/contents.html

And I’m still looking for the 'T'.

[T]he friends we made along the way.

I always thought it was "Techniques"

(PBRT author, here)

pbrt is the Physically Based Ray Tracer that is described in the Physically Based Rendering book.

Thanks for clearing that up!

From some long-ago SIGGRAPH presentations, I recall it as "Physically Based Rendering Toolkit".

I always assumed that PBRT is an acronym for "Physically Based Rendering Textbook".