A diagram is an illustration that describes or explains. It is not simply a picture, but it carries instructions or conveys information about something. It is commonly used in science and mathematics. In British English, diagram may also mean a railway schedule.
There are three adjective forms: diagrammable, diagrammatic, and diagrammatical. The middle form is the most common. Also, the adverb is diagrammatically.
It can also be a verb. To diagram something is to teach a concept by drawing it out in a diagram. The past and progressive tenses may be spelled with one or two ems (diagrammed, diagramed, diagramming, or diagraming). The preferred spelling inside the United States is with two ems.
A sentence may also be diagrammed, which is a way of labeling the parts of speech. Usually this is done as a classroom assignment to teach grammar.