A category that has the same immediate parent as another category within a given hierarchy.
All categories should have at least one sibling, as a category with a single child adds levels to the hierarchy without any increased functionality. Best practice is to keep the sibling categories as balanced as possible by considering how many skus are in each, and how many children each has. Situations in which one sibling node has two children with 50 items in each and the other has 30 children with a total of 2,000 items calls the structure of the taxonomy into question.
Child Category, Parent Category, Triad, Singleton