Bitcoin is the solution to the Byzantine General's problem.

This was once thought to be an unsolvable problem in computer science.

This problem arises in decentralized systems, where it was thought impossible to prove that message sent = message received, since the 'man-in-the-middle' could be a bad actor and falsify the message.

In other words, it seemed impossible to form consensus amongst a network of distributed and independent computers.

By using a timestamp along with a cryptographically secured distributed ledger, Satoshi solved this problem.

His solution is known as Nakamoto's consensus.