An outline is technically a tree. Each node can have zero or more children. Since children cannot link to any node other than their own children, there are no cycles. Since each children cannot refer to their parent, the graph is directed. (Technically, all acyclic graphs must be directed, as a bidirectional link would result in a two-node cycle. But if you don't count those...)
A list would either be a zero or more equal-rank siblings, or a tree where each node had exactly one child. Since that's not how most people take notes, technically they are using a directed acyclic graph rather than a list.
In conclusion, words differ in meaning depending on context :)
