A hash rate(also hash power, hash per second) is the measure of miner's performance (the speed at which a miner solves the Bitcoin code). It is a unit representing the number of double SHA-256 computations performed in one second. Hash rate is used in every cryptocurrency that uses proof of work.
The hash per second is also used in calculations of the Bitcoin network's overall hash rate. The overall hash rate of the network is calculated based on the time between blocks, which is useful when measured over longer periods of time. For the current and historical values of Bitcoin hash rates, see this chart.