In film, a medium shot is a camera shot from a medium distance. The dividing line between "long shot" and "medium shot" is fuzzy, as is the line between "medium shot" and "close-up". In some standard texts and professional references, a full-length view of a human subject is called a medium shot; in this terminology, a shot of the person from the knees up or the waist up is a close-up shot. In other texts, these partial views are called medium shots. (For example, in Europe a medium shot is framed from the waist up.)
There is no evident reason for this variation. It is not a distinction caused by, for example, a difference between TV and film language or 1930s and 1980s language.
Medium shots are relatively poor at showing facial expressions but work well to show body language.
Depending where the characters are placed in the shot, a medium shot is used to represent importance and power.