J2 Tool steel:  A non-sensical term often applied to 420J2 Stainless Steel to make a person think they are buying a better quality knife. There is no such thing as J2 Tool Steel, the seller is referign to 420J2 stainless steel. While it is used to make some tools, it is not truly a "Tool Steel"

Jack: Another name for a knife, especially in nautical circles. In a broad sense, a jack knife is any folding knife in which all the blades open on the top end of the knife. Jack knife is a good generic term to describe any knife that has the blades all located on one end of the knife and does not fall into any of the various recognized patterns. Perhaps the only exception would be a small single blade knife that resemembles an equal end penknife as those are normally called one blade penknives. See also penknife, trapper, folding hunter, copperhead for comparison.

Japanese Steel:  A non-descriptive term for stainless steel manufactured in Japan.  Unless specified it could mean anything.   The dealer is hoping you think it is at least AUS 6 Stainless steel.  A steel manufactured in Japan that is equivalent to 440 A Stainless steel. Unless specified it is meaningless. 

Jasper:  A stone material that comes in a variety of colors including red, yellow and green and may have a speckled appearance similar to turquoise. Because of its speckled nature, it is often incorrectly called red or yellow turquoise.   Jasper literally means spotted or speckled stone.  A certain type of green Jasper is also known as Bloodstone.  Jasper is often used in Native American and new age jewelry and is therefore sometimes used as a knife handle material

Jet Stone:  A stone made of Polished Lignite Coal.  The most common color used is black and thus it is sometimes called Black Jet. It is often used in Native American and new age jewelry and is therefore sometimes used as a knife handle material

Jig (Jigged):  To cut a pattern in to the handle material of a knife handle, normally to give it the appearance of stag.  This is often done with bone. Almost anytype of handle material can be jigged.

Jigged Bone:  a bone handle cut with jigging done to improve grip or imitate stag

Jig-pick or jig-pin: Same tool as a Fly-pick essentially a tempered metal pin used to clean the eye of small lures used in fishing. The eye is the small opening used to attach lure to the fishing line. The fly or jig pick is a common tool found on folding fly-fishing knives. See Flyfisher knife.

Jimping: Small grooves cut in the spine of a blade or knife handle to provide a rough surface for thumb or finger placement and add to blade control. Jimping differs from file work in that jimping's purpose if menat to improve the funcitonality of the knife where as filework is often more decorative in nature. See file work for comparison.