Bitcoin

What is a Satoshi?

Each bitcoin (BTC) is divisible to the 8th decimal place, so each BTC can be split into 100,000,000 units. Each unit of bitcoin, or 0.00000001 bitcoin, is called a satoshi. A Satoshi is the smallest unit of Bitcoin.

How many Satoshis are in a Bitcoin?

There are 100,000,000 satoshi in a BTC.

Why and how Satoshi pick 21 million as the number of bitcoins to be created? What is the significance of that number 21?

A total of 174,100 tonnes of gold have been mined in human history, according to GFMS as of 2012.[2] This is roughly equivalent to 5.6 billion troy ounces or, in terms of volume, about 9261 m3, or a cube 21.0 m on a side.

Since Bitcoin is often compared to gold, total number of bitcoins matches total amount of gold mined in human history which can be imagined as a cube 21 m on a side.