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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'.

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!

[T]he friends we made along the way.

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".