India - Housing Condition and Migration Survey: NSS 49th Round, Schedule 1.2, January - June 1993 

Family nucleus or alternatively nuclear family is the basic family unit consisting of the mother and father with their children. The simplest case is the of a married couple with unmarried children. Each of the following cases will consists of separate family nucleus.
( i ) a married couple with or without their unmarried children.
( ii ) one parent (i.e, father or mother) living with one or more unmarried children.
Children referred to in ( i ) & ( ii ) above may also be widowed, separated or divorced but without any child of their own. If also includes step children and adopted children.
( iii) in the case of a husband having more than one wife ( i.e, case of polygamy) the husband and each wife will constitute one Family nucleus and there will be as many family nuclei as the number of wives.
( iv ) in the case of one wife living with more than one husband in the household (i. e., case of polyandry) the wife with all her husbands together forms one family nucleus.
( v ) in the case of brothers and / or sister(s) who are unmarried, separated, widowed living together, the number of family nuclei is zero.
A single member household does not form a family nucleus. A man and woman living together without formal marriage will be treated as having zero nucleus family. Similarly, friends, cousins etc. staying together will also not constitute a family nucleus. A married couple staying with unmarried friends and relatives etc. will form only one family nucleus. Further a member in a household will be considered only in one family nucleus except in case of polygamy. The number of family nucleus in the household as ascertained following the above concepts will be recorded against this item.