The "thin blue line" is a term that typically refers to the concept of the police as the line which keeps society from descending into violent chaos. The "blue" in "thin blue line" refers to the blue color of the uniforms of many police departments.

The phrase originated as an allusion to the British infantry regiment The Thin Red Line during the Crimean War in 1854, wherein the regiment of Scottish Highlanders — wearing red uniforms — famously held off a Russian cavalry charge. Its use referring specifically to the police was popularized by Los Angeles Police, by which time "thin blue line" was used across the United States and Canada.

Since the late 1970s, the term has also been used in tandem (e.g. "crossing the thin blue line") with the concept of the "blue wall of silence", an informal code among police officers used to cover up police misconduct.

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