A Function is a Term that binds a Variable and produces a result — a body that returns to its context.
Term differentiates into three forms: Variable, Function, and Application. A Function takes a step beyond naming: it binds a Variable (the argument) within a body (the result). When a Function meets an argument through Application, the bound Variable is replaced and the body evaluates.
In the relational derivation, Functions arise because the capacity for self-reference within sequential structure needs not just names (Variables) but operations — Terms that take inputs and produce outputs. A Function is what Composition looks like once it has been formalized: the ability to combine forms by passing one into another.
The term stratum introduces Functions through forming a term body and then forming the function term itself — the body comes first, then the binding that makes it a function.