The term SPY ON can be summarized by two crossword solutions: SURVEIL (7 letters) and PEEPAT (6 letters). SURVEIL is defined as to watch over or observe closely, while PEEPAT is defined as to look at something secretly or furtively. Both terms refer to the act of observing someone or something without their knowledge. This type of activity is often associated with espionage, where a person or organization is trying to gain information without being detected.