A nephew is a son of a person's sibling, and a niece is a daughter of a person's sibling. Conversely, that person is the aunt or uncle of their niece or nephew. The gender-neutral term encompassing both nephews and nieces is nibling. The relationship of aunt/uncle to niece/nephew is an example of second-degree relatives, meaning that their coefficient of relationship is 25%.
The terms are also used colloquially for sons and daughters of siblings-in-law, even though there is no blood relationship.
Nephew and niece Wikipedia
The word nephew is derived from the French word neveu which is derived from the Latin nepotem. The term nepotism, meaning familial loyalty, is derived from this Latin term. Niece entered Middle English from the Old French word nece, which also derives from Latin nepotem. The word nibling is neologism suggested by Samuel Martin in 1951 as a cover term for "nephew or niece"; it is uncommon outside of specialist literature.
Historically, a nephew was the logical recipient of his uncle's inheritance if there was no son or daughter, although in some northern Bangladeshi societies, a nephew takes precedence over a daughter, while in segments of medieval English law, nephews were also at times favored over daughters. In social environments that lacked a stable home or habituation such as refugee situations, uncles and fathers would equally be assigned responsibility for their sons and nephews.
Among parents, some cultures have assigned equal status to daughters and nieces in their social status. This is for instance the case in Indian communities in the Mauritius, and the Thai Nakhon Phanom Province, where the transfer of cultural knowledge such as weaving was distributed equally among daughters, nieces and nieces-in-law by the Tai So community, and some Garifuna people that would transmit languages to their nieces. In some proselytizing communities the term niece was informally extended to include non-related younger female community members as a form of endearment. Among some tribes in Manus Province of Papua New Guinea, women's roles as sisters, daughters and nieces may have taken precedence over their marital status in social importance.
In some cultures and family traditions, it's common to refer to one's first cousin once removed (the child of one's cousin), as a niece or nephew. In archaic terminology, a maternal nephew is called a sister-son, emphasizing the importance as a person's nearest male relative should he have no brothers or sons of his own. Sister-son is used to describe some knights who are nephews to King Arthur and is imitated by J. R. R. Tolkien, especially in lists of Kings of Rohan or dwarves where the sister-son is also heir. Sister-daughter is a less common parallel term for niece.
The terms grandniece (great-niece) and grandnephew (great-nephew) correspond to those of granduncle (great-uncle) and grandaunt (great-aunt), expressing a third-degree relationship.
For (poorly standardized) terminology such as "second granduncle", see first cousins twice removed.