Emission, as the word is used for cryptocurrencies, is covered in full in the entry Token Emission, which explains emission schedules, how they affect supply and price, and how projects design them. This page gives a short definition and explains the shorter name.
Emission is the process by which new units of a coin or token are created and released into circulation. On proof-of-work networks, new coins are emitted as block rewards to miners; Bitcoin's emission halves roughly every four years in the Bitcoin halving and will stop at a cap of 21 million coins. On proof-of-stake networks, new tokens are paid to validators and stakers. DeFi protocols and games often emit tokens as liquidity-mining or user rewards, and tokens held by teams and investors enter circulation through unlock schedules. The emission rate, together with any burning, determines how fast the circulating supply grows, which matters for holders because high emissions can dilute existing holdings, a central concern of tokenomics.
"Emission" and "token emission" mean the same thing; the shorter form is common in trading and analytics dashboards. The word is unrelated to carbon emissions, although some reports on mining discuss both.