system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is "20/20 vision" and why do they call it that? 

What is it that makes a prescription 20/25? Why do we use the term 20/20 to describe perfect vision? 

Edit: Thank you so much to everyone who answered this! Especially to Teotwawki69, excellent answer.
gpt
The top number is an arbitrary designation to indicate what people with "normal" vision can see at 20 feet, and is fixed. The bottom number indicates your vision. So, if you have 20/20 vision, you can see at 20 feet what people with normal vision can. 20/40 means you can see at 20 feet what people with normal vision can see at 40 feet, etc.

This is why a designation like 20/400 generally means legally blind. However, it can rarely go the other way, and there are people with 20/15 vision or better, which means they can see at 20 feet what people with normal vision can see clearly at 20 feet, etc.