An erf (plural erven) is a term used in CRE to describe a piece of land owned by someone (i.e. registered in a deeds registry / office).
The words used for an erf outside of South Africa are plot, lot or stand.
This legal term is used in South Africa, Namibia and Swaziland.
In South Africa the parent of an erf is a township. For a farm, the equivalent of an erf is a farm portion. For an agricultural holding, the equivalent is a holding.
Regarding hierarchical data relationships, one erf can have many “children”. For example, erf 1 can be “parent” to erf 1 portion 1, erf 1 portion 2, erf 1 portion 8, erf 1 portion 10, erf 1 remainder. In turn, a township “parent” can be made up of many erf “children”.
The term is Dutch in origin.
Section 102 of the South African Deeds Registries Act, 1937  provides the following definition:
“erf” means every piece of land registered as an erf, lot, plot or stand in a deeds registry, and includes every defined portion, not intended to be a public place, of a piece of land laid out as a township, whether or not it has been formally recognized, approved or proclaimed as such.