Application specific integrated circuits or ASICs are mining hardware designs that are considered to be the pinnacle of Bitcoin mining equipment. The design of an ASIC trades off all computing functionality to be efficient at only one task: mining, or more specifically: hashing. ASICs are highly customized equipment and are expensive to produce. They can vary widely in energy efficiency and overall hashing output, with general technology advances in chip fabrication leading the way to better ASICs.
The presence of ASICs as participants in the Bitcoin network has been criticized as being a centralizing force. Individual ASIC manufacturers or a small number of people with sufficiently large amounts of capital to risk can command a very high percentage of the network hashing power. Although this critique is quite widespread, no solid solution to the issue has been proposed.