ASIC is the abbreviation for 'application-specific integrated circuit'. These are microchips or processors, designed to perform a very specific task very well. The Bitcoin ASIC is a very popular one, which makes it possible to mine Bitcoins very efficiently, making it impossible to mine them on a normal computer. ASICs are also developed for other coins, but they can become useless after a protocol upgrade, that makes the blockchain 'ASIC-resistant'.