An application-specific integrated circuit (ASIC) is a circuit that was created with a single purpose in mind. For example, a Bitcoin mining ASIC is a machine with microprocessors devoted specifically to mining Bitcoin. Other cryptocurrencies like Etheruem and Dash will have their own version of ASICs.
Rather than being general-purpose integrated circuits like RAM chips or microprocessors for computers or mobile devices, ASICs are specialized integrated circuits used in cryptocurrency mining only. That makes them more efficient at mining than general-purpose computers and that’s why mining with ASICs is more profitable than using CPUs or graphics processing units (GPUs).