J
Jambiya
The Arab knife, found in every country the Arabs have lived in. Strongly curved blade, double edged with a rib in the middle. Each country has a somewhat different version.
Jack knife
A knife with one or two blades larger than a pen knife, smaller than an hunter
Japanese Blades
Instead of naming the different blades separately I have decided to list them all here. Jin Tachi, the longest, from about 33 inches. Katana and Tachi 24 to 30 inches. Wakizashi 16 to 20 inches, the Tanto and Aikuchi with lengths of 11 to 16 inches and the Yoroi Toshi having blades of 9-12 inches and the Kwaiken with blades of 3 to 6 inches. A really good understanding of the blades of Japan requires more study than that of all other knives combined. We can only offer the simplest terms.
Jigged Bone
Bone that has had the surface cut to give a textured finish. Originally done to imitate deer antler, then in many different textures just for beauty and to give a better grip.
Jumbo (handle shape)
Oversized folding knife handle, one end larger than the other