A computer that contains an ASIC chip, which is then used to mine bitcoin. They may connect directly to a computer or network wirelessly or with the use of an ethernet cable.
A computer that uses ASIC for mining is much faster than using any normal desktop or laptop. Essentially, an ASIC is a chip specifically created to execute one task. An example of one such model is an ASIC miner created to ONLY process SHA-256, which is the problem offered by the Bitcoin Blockchain to mine new coins. There are also ASICs for scrypt which specifically solves the mathematical code in relation to altcoins, such as litecoin. However, in recent years there has been a good amount of dialogue surrounding the longevity of this method of mining since there has been an emeregence of coins that make it impossible to mine with an ASIC.