1. past tense and past participle of spy; to watch secretly or gather information covertly
2. past tense and past participle of spy; to catch sight of or notice suddenly

Examples

• The detective spied on the suspect from across the street.
• Government agents spied on foreign diplomats during the meeting.
• She spied her friend in the crowded mall.
• He spied a rare bird through his binoculars.
• The children spied on their neighbors from the treehouse.
• I spied an error in the document while proofreading.
• The scout spied the enemy camp from the hilltop.