Refers to a physically long sword, often in contrast to a short sword (短剣). Used in historical, fantasy, or descriptive contexts.
The knight wore a long sword at his waist.
Used for the minute hand of a clock, in contrast to the hour hand (短剣). This is a specialized, somewhat dated term; modern Japanese more commonly uses 長針 (ちょうしん).
See also: 短剣 (たんけん)
In old clock manuals, the minute hand is sometimes written as 長剣.