Same as defined on Bitcoin whitepaper.

I see that people wanting to redefine Bitcoin names are probably British or from former-brittish colonies. They are used to redefine "feet" every time a new king comes with a q different shoe size, or than move to miles and knots, and for weight using ounce, stones, etc.

Than some clever person who knows math comes and say: we dont need new names, just add a prefix!

So, 1 nano BTC = BTC/1000000000

1 pico BTC = BTC/1000000000000

1 Femto BTC= BTC/1000000000000000

https://en.m.wikipedia.org/wiki/Metric_prefix

This easy prefix system is just 226 years old!

For some history:

In the middle of the 19th century, James Clerk Maxwell conceived a coherent system where a small number of units of measure were defined as base units, and all other units of measure, called derived units, were defined in terms of the base units.

https://en.m.wikipedia.org/wiki/History_of_the_metric_system