Ethereum halving is the process whereby the block reward for mining Ethereum is reduced by 50%. This event occurs every 4 years and results in a reduction in the amount of new ETH being created.
The halving is designed to keep the supply of ETH inflationary, which in turn should maintain or increase the value of ETH over time. The next Ethereum halving is scheduled to occur in 2020.
The impact of Ethereum halving on the price of ETH is uncertain. Some believe that it will have a positive effect as it will reduce the supply of new ETH and thus increase demand and price.
Others believe that it will have a negative effect as miners will be incentivized to sell their existing ETH in order to recoup lost earnings. Only time will tell what the effect of Ethereum halving will be on the price of ETH.