Definition: Due Diligence is a legal concept that relates to whether a reasonable individual under similar circumstances would have taken similar action under the same circumstances.
For a business and organization, as it relates to computer security, did the question is "Did you take reasonable and prudent steps toward providing for the security of the information? Did the organization have security best practices in place? Organizations could, otherwise, be sued for either revealing confidential information (personal identities, credit card numbers, private healthcare information) if weak security is found to be a contributor to the loss of confidentiality. The court would want to know did the defendant perform his or her homework and provide for reasonable and normal security practices or was there failure to properly discharge fiduciary responsibilities. Various states and countries now have laws related to computer security requirements.
Its Relevance: Legitimate transactions over the Internet must be assured business to business and business to customer commerce would come to a halt. Providing for the assurance of customer and seller identity is, therefore, vital.