Security Incident and Event Management (SIEM) is a term or acronym related to Cybersecurity that also includes several variants; SEM, SIM (often used interchangeably) which refer to real-time monitoring, collection, and correlation of cybersecurity events, notifications, and console views for the purpose of identifying cybersecurity threats events and threats. Long-term storage and analysis, manipulation, and reporting of collected log data and security events records is required, which is known as security information management (SIM) and as you can infer by putting this together, you have security information and event management or (SIEM).
SEIM data is typically monitored by a Security Operations Center or (SOC) which can provide Incident Response to detected cyber threats.