An ASIC (application-specific integrated circuit) miner is a device specialised only for a specific type of work. In the cryptocurrency space, ASICs are capable of hashing certain mining algorithms (e.g. SHA-256 on Bitcoin or Equihash on Zcash) with high efficiency.
These workers represent an upgrade from CPU/GPU chips, which usually become obsolete for any given mining algorithm after ASICs are developed and introduced to the market.
Visit asicminervalue.com for a comprehensive list of ASIC miners for different coins.