Can someone explain the meaning of 'proxied by' in math? or What does 'proxied by' mean in mathematic

Wow, thanks everyone! This makes way more sense now.

I didn’t realize it was such a common trick in stats and optimization.

Gonna check out those links and videos—especially the stats glossary.

Follow-up: How do you know if a proxy is *good enough*? Like, what makes a bad proxy vs. a decent one?