A condition due to a deficiency of one or more essential vitamins.

Avitaminosis may be qualified with the letter of the deficient vitamin, e.g. "avitaminosis A". Hypovitaminosis is often given as a synonym; sometimes a distinction is drawn betwixt the two, in which avitaminosis is defined as a comparatively severe disease resulting from long-term deficiencies (see list below), while hypovitaminosis is a milder deficiency, great enough to effect health, but not so great as to cause a severe disease. 'Avitaminosis' is the more commonly used of these two terms (as judged by a Google search). The opposite, having too much of a vitamin, is called hypervitaminosis.

Some avitaminotic diseases include:

Vitamin deficiencies can lead to other conditions, such as dermatitis, anaemia, cheilosis, atherosclerosis, and mood disorders, which are not unique to people suffering to vitamin deficiencies. Avitaminosis refers to the cause, not specific results.