A Satoshi is the smallest unit of Bitcoin recorded on the Blockchain — a bit like the cent is the smallest tradable value of the United States Dollar. It carries a value of 0.00000001 BTC, one-hundred-millionth of a single Bitcoin.
All Bitcoin transactions are denominated in Satoshi, which is shortened to SAT, before being converted to the Bitcoin equivalent for illustration purposes on the front-end, so as not to confuse customers who may not be aware of the Satoshi and its worth.
Since the value of a Satoshi is so low, it's rare you'll find a retailer seeking remuneration in the form of one. Instead, they prefer to illustrate all figures, regardless of how small or large, in BTC — referring to 0.00000001 BTC as exactly that, rather than 1 SAT.
Granted, if the value of Bitcoin, and subsequently all denominations, surges to such a level — as some analysts predict will be the case — where one Satoshi is equal to a single dollar (1 USD), there's a good chance the term will make its way into the mainstream.
Want to learn more about Cryptocurrency? Check out our Beginner's Guide.