DKIM - DomainKeys Identified Mail
DKIM or DomainKeys Identified Mail is a security measurement to check if an email is sent from authorized or unauthorized sender.
It adds a digital signature to every message sent. This allows the receiving server can verify if the message is forged.
A sender can pretend the message is sent from abcbank.com, for example. You might see the sender email is email@example.com, but actually it is sent from malicious source. Checking the DKIM, will reveal if this email is indeed sent from a specific domain, that is authorized by the domain's owner.
2 other terms related to DKIM are SPF (Sender Policy Framework) and DMARC (Domain-based Message Authentication, Reporting, and Conformance).