Hardware optimized to solve a specific algorithm for the purposes of mining cryptocurrency.
ASICs can output significantly more hash power than consumer hardware, giving them a massive advantage. ASICs are so much more powerful than consumer hardware, that they are often the only profitable method off mining.
ASICs can be utilized when mining for cryptocurrencies with consistent algorithms, but they can be designed and optimized for any set of algorithms. Some crypto assets attempt to be ASIC resistant by changing their algorithms in random patterns; but he effectiveness of this is yet to be determined, with enough time everything can be optimized.