A data breach is a security incident, in which an unauthorized actor gets access to sensitive or confidential information (and then typically proceeds to copy, view, or steal the data). Data breaches have been perpetrated by means of hacking, malware, phishing attacks, or insider action.
For organizations, a data breach could result in monetary losses, negative press coverage, and legal liabilities. For the individual whose data was leaked or stolen, it could mean falling victim to identity theft or financial fraud.
The aftermath of a data breach is determined by an organization's incident response plan. Common response actions would be notifying affected individuals, investigating the source of the breach, and remediating any vulnerabilities that the attacker had exploited.