Def: a **Series** of type $(U,V)$ consists of an index of type $U$, which provides the sequence of axis labels, along with a matching sequence of values of type $V$.
$U$ and $V$ are Python types, which include the primitive value types, string, tuple, and object.
A Series also has an optional Name attribute.
Caveat: the type $U$ must be "hashable." That includes all of the types that we might normally consider for axis labels, such as integers, strings, or tuples of strings.