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ERC20 is a technical standard used for smart contracts on the Ethereum blockchain for implementing tokens. ERC stands for Ethereum Request for Comment, and 20 is the number that was assigned to this request. The clear majority of tokens issued on the Ethereum blockchain are ERC20 compliant.
New coins and tokens created on the Ethereum blockchain are usually created by the ERC20 standards. So often tokens by default are referred to ERC20 tokens by the crypto community.