1. close observation of a person or group, especially one under suspicion
2. the act of carefully watching and monitoring an area or activity for security purposes

Examples

• The police kept the suspect under surveillance for weeks.
• Security cameras provide surveillance of the parking lot.
• Government surveillance programs monitor internet communications.
• The store uses surveillance equipment to prevent theft.
• Medical surveillance helps track disease outbreaks.
• Electronic surveillance has become more sophisticated over time.
• The building’s surveillance system recorded the incident.