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Uncle (from Latin: avunculus "little grandfather", the diminutive of avus "grandfather") is a family relationship or kinship, between a person and his or her parent's brother or parent's brother-in-law. A biological uncle is a second degree relative and shares 25% genetic overlap. A half-brother of one's parent is a half-uncle who is a third-degree relative with 12.5% genetic overlap.
A granduncle (sometimes written as great-uncle or grand-uncle) is the brother, brother-in-law of one's grandparent. Although the term "great uncle/aunt" is often used, it is, strictly speaking, incorrect. The naming of the siblings of one's grandparents and great grandparents follows the simple logic of how the grandparents are named. Thus, the siblings of one's grandparents are grand uncles and grand aunts and the siblings of one's great grand parents are great grand uncles and great grand aunts. Using the grand terminology is also consistent with how nephews and nieces are named. One does not refer to great nephews and nieces, so to be consistent why use "great" in conjunction with uncles and aunts.1 
In some cultures and families, children may refer to the cousins of their parents as "aunt" or "uncle". It is also a title of respect for elders (for example older cousins, neighbors, acquaintances, as well as total strangers). See fictive kinship.
Also in some cultures, like Slavic or Persian cultures, no single inclusive term describing both a person's kinship to their parental male sibling or parental male in-law exists. Instead there are specific terms describing a person's kinship to their mother's brother (termed "daiyee") or a person's kinship to their father's brother (termed "amou").
An analogous differentiation exists in modern Persian using separate terms to describe a person's kinship to their mother's female sibling (termed "khaleh" ), and a person's kinship to their father's female sibling (termed "ammeh").
Furthermore in Persian culture the terms used to describe a person's kinship to their maternal or paternal in-laws bear clear and unambiguous descriptions of that relationship, thus serving the purpose of differentiating the parental in-laws from blood-relatives and giving an indication of the type of relationship. For example there exists a specific term describing a person's kinship to the spouse of their paternal uncle (i.e. "zan-amou" literally 'wife-of-' amou). Thus a distinction is made that kinship is to the spouse of the person's paternal male sibling (and as such is not a blood-relationship ).
Uncles and Aunts are considered important in modern Irish culture and are usually chosen to be godfather or godmother of children during Catholic baptism. A young Irish person might seek the counsel of their favourite aunt or uncle before making an important decision and the opinion of the respective aunt or uncle is treated seriously.