Bitcoin Hash Rate
A hash is the output of a hash function and, as it relates to Bitcoin, the Hash Rate is the speed at which a compute is completing an operation in the Bitcoin code. A higher hash rate is better when mining as it increases your opportunity of finding the next block and receiving the reward.
The hash rate is the measuring unit of the processing power of the Bitcoin network. The Bitcoin network must make intensive mathematical operations for security purposes. When the network reached a hash rate of 10 Th/s, it meant it could make 10 trillion calculations per second.
Mhash/s = millions hashes per second (double sha256 raw speed performance; may not be very energy efficient with some models)
Mhash/J = millions hashes per joule (energy efficiency; 1 joule of energy is 1 watt during 1 second: 1 J = 1 W*s)
W = watt (maximum power consumption, i.e. energy per unit of time: 1 W = 1 J/s)