(from Google Dictionary)
1. a set of things working together as parts of a mechanism or an interconnecting network. e.g. railroad system
synonyms: structure, organization, order, arrangement, complex, apparatus, network
(from J. Christopher Jones)
1. interconnected elements with a purpose
Your TOC is a map for your reader.
It's also a tool you can use to organize your thinking and tell the story of your work.
Organizing (What kinds of content are you working with?)
Grouping (What goes together?)
Titling (Name it! Apply meaning.)
Sequencing (Order it. Understanding unfolds over time.)