Translation guide
The iconic sword associated with ninja, often shorter and straighter than a katana, used for stealth and utility.
ninja sword
Most common term; also called 忍刀 (しのびがたな) in some contexts.
A ninja sword is shorter and straighter than a regular katana.
An alternative term, literally 'stealth sword'. Slightly more traditional or literary feel.
The shinobigatana is a sword said to have been used by ninja.
Discussing the actual historical weapon, which may differ from pop culture depictions
Even in historical discussions, 忍者刀 is the standard term, though historians may note that its exact form is debated.
There are various theories about the shape of historical ninja swords.
Referring to the weapon in movies, anime, games, etc.
The go-to term in any modern context.
The main character of that anime carries a ninja sword on his back.
While often called a 'ninja sword' in English, it is distinct from a katana. In Japanese, 刀 (かたな) is a broad term, but 忍者刀 specifically implies the ninja version. Avoid calling it 忍者刀 when you mean a regular katana.