Comment by noisy_boy

Edit: Found it - its /usr/share/units/definitions.units (on Pop!_OS, so probably same on Ubuntu/Debian).

The FAQ[0] mentions:

> units on SunOS doesn't know about "millilightseconds."

> Yes. So? I used to populate my units.dat file with tons of extra prefixes and units. And actually, I think I was using AIX to run units; I don't know if it knew about millilightseconds. Take a look at the units.dat shipped with Linux these days. It definitely knows about millilightseconds.

I tried locate for units.dat but couldn't find it. Anyone knows where is it? Not keen on running a system-wide find.

[0]: https://www.ibiblio.org/harris/500milemail-faq.html