What is the X in X-mas?
A data type composed of multiple elements. An aggregate can be homogeneous (all elements have the same type) e.g. an array, a list in a functional language, a string of characters, a file; or it can be heterogeneous (elements can have different types) e.g. a structure. In most languages aggregates can contain elements which are themselves aggregates. e.g. a list of lists.
See also union.