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RSA
Abbreviation for Rivest-Shamir-Adleman
In short and in plain language
RSA is a widely used way to lock data and to sign it.
It is named after the people who made it.
In email, it is used, for example, to sign the mail you send out. The length of the key is part of what makes this safe.
Definition
RSA is a widely used method for encryption and digital signatures, named after its inventors. In email it is used, for example, to sign outgoing messages with DKIM. The key length helps determine how secure the signature is.