How is π.com a valid domain name?

The startup Physical Intelligence has a domain of π.com, which redirects to/from https://www.xn--1xa.com

How is this character represented in the domain name?

that is really cool

When I worked for an ISP in the early 90s, we were just getting accustomed to real DNS domain names being rather ubiquitous and properly distributed.

My supervisor (who became an avid venture capitalist/angel investor) burst into the NOC one day to crow about having purchased “sex.org” and he knew damn well he would never use it; he was just getting in on the speculation.

Anyway, we had a real customer you may know as 3M. Now most companies are pleased to register a quite readable and recognizable domain name that evokes their company, and so you can perhaps imagine how miffed 3M’s execs were, when they found out that “3M.COM” was literally illegal under IETF standards!

Now I was also forced to imagine this, because I certainly was never privy to such meetings or the decision process, but the outcome was plain for all to see:

3M chose to register the domain name “mmm.com” and so for many, many years at the dawn of the Information Age, their public-facing domain and brand was a wry comment on the narrow shortsightedness of Internet standards, and clever ways to get around them.