An ASIC, or application-specific integrated circuit, is an integrated circuit (fancy word for for computer chip) customized for a particular use. In the case of Bitcoin, ASICs are microchips specifically designed for Bitcoin’s SHA-256 algorithm.
Originally, Bitcoin was mined with ordinary central processing unit (CPUs), and then specialized graphics cards (GPUs). Both CPUs and GPUs are found in regular home PCs, and ar particularly better in gaming computers.
As competition for mining bitcoin increased, people started to design ASICs specifically for Bitcoin’s SHA-256 hashing algorithm.