BTC, Bitcoin (BTC)
Bitcoin (BTC) is the bitcoin implementation with the most security, and hash power; a store of digital value. Bitcoin allows anyone to send BTC to anyone in a trustless and secure manner.
The most popular implementation of bitcoin, aiming to bring peer 2 peer cash to the world while keep the blockchain ledger size as small as reasonable. Keeping the ledger small ensures everyone can run, verify and help secure the blockchain without trusting any third parties.
Keeping the ledger small makes the bitcoin network extremely difficult to censor and control, as the data requirements are small enough to get through any barriers or censorship.
When a merchant 'Accepts Bitcoin' their payment point of sale often will accept other cryptocurrencies such as Bitcoin Cash (BCH), Ethereum (ETH), Litecoin (LTC) and others. Try them all out when possible to learn the differences and experience using cryptocurrency as currency.
* All terms and definitions may update as the Cryptionary improves.