It is one of the seven basic physiological principles . The word khilt (Humour) literally means an admixture. As blood phlegm, yellow bile & black bile are intermixed inside the blood vessels that is why, these are known as “Humours”. And this admixture is known as blood because blood content use to be more in this mixture as compared to other humours and the colour of the mixture is dark red .But these humours are of different types and properties and serve different functions.

There is some sort of difference of opinion between the definitions of Akhlat (humours) given by different physicians of these the following definition seems to be more appropriate in view of the diverse properties and functions of Akhlat.

“Akhlat are the moist and fluid parts of the body which are produced after trans formation and metabolism of ailments, they serve the function of nutrition, growth and repair and produce energy for the preservation of individual and his species. A right proportion and intermixture (homeostasis) of them, according to quantity and quality constitutes health and up right proportion or imbalance (Su,Al-Mizaj) according to quantity and quality and irregular distribution leads to disease.

The father of medicine Hippocrates (460.B.C) postulated the humeral theory., According to him-“The body contains four major kinds of humours dam (blood), balgham (phlegm) safra (yellowbile) and sauda (black bile); a right proportion, according to quality and quantity and mixing of which i.e. homeostasis constitutes health and un right proportion and irregular distribution, according to their quantity and quality constitutes disease.

The term Akhlat applies to all fluid of the body irrespective of their colour, location, quantity and quality. Akhlat are not only four in number. The four major humours, blood ; phlegm; yellow bile and black bile can further be subdivided.

CLASSIFICATION OF AKHLAT (HUMOURS)
Various criteria have been adopted in Tibb for the classification of akhlat, which are as under.
According to their locations.
According to their colours.
According to their usefulness.
According to being primary or secondary fluids.
According to their quality of fineness and coarseness.
According to their quality of being normal and abnormal.

SANGUIS- BLOOD (AL-KHILT AL-DAM)
Dam (blood) is regarded as mixture of all four kinds of Akhlat i.e. dam, balgham, safra and sauda. But since the red colour is dominating hence, the whole mixture is called as dam (blood).
The temperament of blood is hot and moist.

PHLEGM (AL- KHILT AL BALGHAM)
All the white or colourless fluids of the body are called as balgham (phlegm) the temperament of balgham is cold and moist.

YELLOW BILE (AL- KHILT AL- SAFRA)
All the yellow fluids (and compounds) of the body are called as safra and those fluids are also called as safra which express the signs and symptoms attributed to safra.
The temperament of safra is hot and dry.

BLACK BILE (AL- KHILT AL-SAUDA):
The most inferior among all the Akhlat (humours) is sauda. Position of sauda is next to safra.
The temperament of sauda is cold and dry.