A person of mixed race. In Hindu religion, there are four Varn; or the society is divided into four types of people to manage the whole society, they are given here according to their status in the society -

(1) Braahman - whose job is to study and teach, to do Yagya for themselves and others, and to take and give donations.
(2) Kshatriya - whose job is to protect the society from enemies. This is a warrior class and their duty is keep discipline in the society and punishing the criminals.
(3) Vaishya - whose job is to do farming, trading and cattle rearing to feed the society.
(4) Shoodra - whose job is to serve the other three Varn of the society.

Braahman are supposed to be like Devtaa, lower than them are Kshatriya and lower than them are Vaishya and the lowest than the Vaishya are the Shoodra. Marriages should normally be performed among their own Varn, but 

a Braahman boy can marry a Braahman or Kshatriya or Vaishya girl; 
a Kshatriya boy can marry a Kshatriya or Vaishya girl;
a Vaishya boy can marry only a Vaishya girl.

--When a marriage is performed between two Varn people, their children are called Varnsankar.
--When a marriage is performed between a higher Varn girl and a lower Varn boy, their children are called Soot.
--When marriages between two Varn are common in the society, its organization is supposed to be in danger, that is why olden people used to avoid it.

Padm Puraan, 3/5  (p 391) mentions 11 types of activities which instigate Varnsankar situations -