Comment by gregw2

Update: Found the term (with dash) used in SIGGRAPH 1988 course notes from Don Greenberg: "Physically-based rendering methods: A Radiosity Approach":

https://books.google.com/books?id=Vw1DAQAAIAAJ&q=%22physical...

I see the term without the dash used in 1994 in Eurographics Computer Forum "A Theoretical Framework for Physically Based Rendering": https://www.cs.rpi.edu/~cutler/classes/advancedgraphics/S11/...

In 1994 siggraph paper by Greg Ward, https://radsite.lbl.gov/radiance/papers/sg94.1/Siggraph1994a..., the term is explicitly described in a footnote:

'The term "physically-based rendering" is used throughout the paper to refer to rendering techniques based on physical principles of light behavior for local and global illumination.'

I defer of course to mattpharr if he cares to comment! :)