The act or process of extending the validity of the data binding asserted by a public key certificate by issuing a new certificate. (The new certificate is typically used to replace the existing certificate, and both certificates typically contain the same Subject DN and SAN information. It is best practice to generate a new key pair and CSR, i.e., re-key, when renewing a certificate, but re-keying is not required by all certificate authorities. Renewal is typically driven by the expiration of the existing certificate but could also be triggered by a suspected private key compromise or other event requiring the existing certificate to be revoked.)
Source(s):
NIST SP 1800-16B
under Renew
NIST SP 1800-16C
under Renew