The mining reward for bitcoin has just now been cut from 25BTC ($15k) per block to 12.5BTC ($7.5k) per block. Right at the time of the halving the price suddenly dropped $30 to $630. Every 4 years the mining reward for bitcoin is halved.
The mining reward halving is very interesting compared to other national currencies. While national currencies are perpetually having their supply increase (excuses given : stimulate the economy, bailouts etc) rather the rate of bitcoin supply is decreasing. While national currencies have the dilution of their value built into their mechanism rather bitcoin has concentration of value built into its mechanism.