Double-spending is a unique problem associated with digital currencies, that allows for an online value unit to be spent more than once. This is because the digital information surrounding the value unit can be reproduced relatively easily by tech savvy individuals in absence of a central authority that keeps tab of each and every transaction between parties.

Bitcoin, the first commercial application of Blockchain technology, solved this problem by preventing Double-spending on digital value units that are created online, through a unique cryptographically executed process and recording transactions in a distributed ledger needing consensus from all parties before they are finalized and approved.

Hence it is called a Cryptocurrency. Bitcoin employs a universal ledger, termed blockchain and provides a way for all nodes to be aware of every transaction confirmed through a game theory inspired process and broadcasted to all the nodes in the form of blocks.