The speed at which new coins are produced and released.
As an example, let’s take a look at Bitcoin. A new block is added to its blockchain every 10 minutes. When the cryptocurrency first launched, miners were rewarded with 50 BTC for every block that was validated — meaning that the emission rate of BTC was approximately 7,200 a day.
A series of halving events have taken place over recent years, meaning that the number of new Bitcoin that’s entering the ecosystem has decreased substantially. As of May 2020, this stands at just 6.25 BTC.
Emissions aren’t guaranteed to continue forever, and in Bitcoin’s case, the last-ever BTC will be mined in 2140 because it has a maximum supply of 21 million and a set schedule for release.
Some cryptocurrencies have no set rate of emission, meaning that new units can be created on demand. A good example of this is the Tether stablecoin, which is created whenever someone places $1 in reserve.