spy
This term carries a heavy connotation of secrecy, deception, and betrayal. While it can be used casually to describe children playing or curious neighbors, in a professional or political context, it implies a high-stakes breach of trust and the clandestine acquisition of sensitive data. As a noun, the word is standardly countable when referring to an individual agent. However, when referring to the general practice of gathering intelligence, it functions as an uncountable concept, though the term espionage is more common for the abstract activity.
Meanings
A person who secretly collects information about an enemy government or organization.
"The agency recruited a double spy to infiltrate the foreign embassy."
To secretly observe someone or their activities to gather information.
"He suspected that his competitors were trying to spy on his business operations."