By definition the word itself means impurity / fault / blemish / vitiator and so on. However, according to the ancient Ayurvedic literature the term dosha has three important meanings / functions. Sharng. 1.5.23-24 Gives the three functions of dosha:

Vata, pitta, and kapha are called doshas (blemishes, vitiators), dhatus (supports, tissues), and malas (wastes) in different contexts: doshas because they vitiate the body, dhatus because they support the body, and malas because they contaminate it.

In general these doshas perform the following functions:

Each dosha has a physical nature composed of 5 elements but predominately of two:

Vata is constituted predominately of space and air elements; pitta is made of fire and water elements, predominately; kapha is composed of primarily water and earth elements. In the modern periodic table of elements (comprised of 92 naturally occurring elements or 118 including the artificially created elements) the various individual entities that make up all substances are reduced by the Ayurvedists to 5: space/ether, air, fire, water, and earth. These may be viewed as states of energy which equate to the most rarified forms to those having high density elements or mass.

Each dosha has a special functional nature which derives from this predominance of two elements (the exact nature of which varies according to textual source:

Each dosha have five special functions or sub-types:

Each dosha has a primary site of function and wherein it accumulates and from which it spreads--called seat of the dosha and numerous secondary sites:
According to Caraka Su. XX.8 urinary bladder, colon, waist, thighs, feet, bones, and especially the colon are the sites of vata; sweat, plasma and lymph, lasika (sebaceous secretions), blood and stomach are pitta’s sites with the stomach its main site; kapha resides in the chest, head, neck, joints, stomach, fat but especially the chest.

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