In the martial arts it is customary to define a strike, block, or kick as targeting a specific general area of the body. At times these references may include a particular body part, for example Atama Mawashi Geri might indicate a roundhouse kick to the head.
At other times these motions might simply target a more general area of the body. The most common way to make such references is to say a strike or block is targeting a high, middle, or low area of the body. Usually these references are made in terms of Japanese terminology.
The term Jodan simply means “high level.” When the body is the subject of discussion then high level is generally thought to be a location at shoulder level or higher. This would include the shoulders, throat, neck, face, and head.
A reference such as Jodan Gyaku Tsuki would indicate a Gyaku Tsuki targeting any area of the body at or above the shoulder level. The strike might impact any of the areas of body mentioned earlier. So the specific target is not identified. The only thing that is relevant is that the strike should be delivered at a high level.
This term refers to something that is a mid-level. When referencing the body this would suggest any part of the body from just below the shoulders to the groin. It is in essence the trunk or torso of the body.
Like Jodan, the term Chudan does not indicate a specific target but rather an area of the body. A Chudan Yoko Geri might strike anything including the kidneys, abdomen, spine, groin, or ribs.
A Gedan level strike or block would indicate something that generally is targeting something below the hips. You might be performing a Gedan Barai or you might be delivering a Gedan Mae Geri. So once again the term is used only to suggest that the specific action is targeting a lower area of the body.
These terms are not absolute. It is entirely possible that a Chudan strike might impact the shoulders. A Jodan strike might impact the upper chest. And a Gedan strike might be targeting the groin or lower abdomen. So there is some overlap in these definitions. They are not fixed or defined areas of the body.
Think of these terms a meaning high (Jodan), middle (Chudan), and low (Gedan). If used in English the term “high” would not suggest that a strike should be limited to the shoulders and above. The term would suggest only that you should strike up high on the body somewhere. This is all these Japanese references mean as well. They simply are direct equivalents of high, middle, and low.
The terms Jodan, Chudan, and Gedan are also used in other contexts as well. The upper floor of a building might be referred to as Jodan level. A bottom floor might be referred to as Gedan level. A high ranking executive might be thought of as a Jodan manager while a first level manager might be a Gedan level manager.
The terms can also be applied to weapons usage. A weapon held at Jodan level is generally positioned such that the business end of the weapon is positioned high in the air (not necessary high over the head). In Gedan position the weapon may be lowered such that it generally points to the ground, or perhaps in the direction of an opponent’s feet. At Chudan level the weapon might be pointed directly at the opponent’s torso and its many vital organs.