| noun | keeping a secret or furtive watch |
| noun | the act of keeping a secret watch for intelligence purposes |
| noun | the act of detecting something; catching sight of something |
| verb | catch sight of |
| verb | watch, observe, or inquire secretly |
| verb | catch sight of; to perceive with the eyes “he caught sight of the king's men coming over the ridge” |
| verb | secretly collect sensitive or classified information; engage in espionage “spy for the Russians” |
| spy | verb | watch, observe, or inquire secretly |
| detect | verb | discover or determine the existence, presence, or fact of |
| espy | verb | catch sight of |
| spot | verb | catch sight of |
| spotter | noun | a person employed to keep watch for some anticipated event |
| spotting | noun | the act of detecting something; catching sight of something |
| espial | noun | the act of detecting something; catching sight of something |
| spying | noun | the act of detecting something; catching sight of something |
| spy | noun | (military) a secret agent hired by a state to obtain information about its enemies or by a business to obtain industrial secrets from competitors |
| snooper | noun | a spy who makes uninvited inquiries into the private affairs of others |
| snoop | noun | a spy who makes uninvited inquiries into the private affairs of others |
| sleuthing | noun | a police investigation to determine the perpetrator |
| sleuth | noun | a detective who follows a trail |
| sight | noun | the act of looking or seeing or observing |
| sighting | noun | the act of observing |