Definition
Bitcoin is the first decentralized digital currency, where a worldwide network of computers records and verifies a ledger of transactions together, with no central bank or administrator. It isn't a physical coin or bill. It exists only as data on the internet and runs on computer code and mathematical rules. Note that under Korean law, it isn't treated as money (legal tender) but as a "virtual asset."
How do you send money without a bank?
When you do a bank transfer, you go through a trusted middleman: the bank. The bank's system writes in the official ledger, "Took 10,000 won from A's account and put 10,000 won into B's account." That's why you can trust each other and trade.
Bitcoin removed that middleman, the bank, from the system entirely. Instead, it had countless computers (nodes) around the world each keep an identical copy of the transaction ledger. When someone sends coins to another person, that transaction is broadcast to the whole network, and many computers each record it the same way in their own ledgers.
Thanks to this structure where everyone shares the ledger with no middleman, the whole system keeps running even if some computers (nodes) shut down or get attacked.
How does it stop fake money and double spending?
A photo or document on a computer can be copied and sent to as many people as you like. If digital money could be copied the same way, and sent to two people at once (double spending), nobody would trust it.
To prevent this, Bitcoin gathers the transactions that happen about every 10 minutes into one box (a block) and locks it with a strong cryptographic lock. To put on this lock, computers around the world have to use huge amounts of computing power to solve a complex math puzzle. This hard work is called proof of work, or mining.
Only the computer that solves the puzzle first gets to attach the new box of transactions to the existing chain of ledgers. To secretly change a past ledger that's already attached, you'd have to redo all the puzzles stacked on top of it, faster than a majority of the world's computers. So in practice, forging or tampering with transactions is extremely hard.
A Bit More Precisely: Digital Scarcity Built on Rules
With ordinary national currencies, a central bank can decide to increase the money supply depending on the economy. If too much money gets released, money loses value. That's inflation. But Bitcoin can't be issued more on someone's whim. Its program code caps the total at 21 million coins.
There's also an automatic adjustment called the halving. About every four years, the amount of new coins released through mining is cut in half. Like gold, the total supply is fixed, and it's designed to get harder to dig out over time. That's how digital assets were given scarcity.
In the end, Bitcoin is a new kind of currency experiment. Instead of the promises of a state or those in power, it builds trust with open math and cryptography that anyone can verify.
Common misconceptions
Bitcoin is completely anonymous, so you can never find out who a criminal is.
Every Bitcoin transaction and wallet address is openly visible on the worldwide ledger. The moment a wallet address is linked to a real person, all of their past transactions can be traced at a glance. So Bitcoin is closer to pseudonymous than fully anonymous.
Where you meet it
It's sometimes used to send value directly over the internet to a friend abroad, without going through a bank. But fees rise when the network gets busy, and the person who receives it still has to convert it into local money separately.
In countries in an economic crisis where the local currency is crashing, it's sometimes used as an alternative currency or a way to store value.
In one sentence
A digital currency with a limited supply, where participants around the world share and write the ledger together, and mathematical cryptography prevents forgery, all without a central bank.