Subnode

A subnode is any piece of content that an [agora] user contributes; most commonly as of the time of writing ([2020-11-17]) a note in Markdown format, but eventually very likely any piece of media.
Subnodes are the children of nodes, which are the heart of the [agora].
What you are reading right now is a [subnode]. If you look below or above, you will see other notes taken by other users -- these are all different subnodes in the same node, which happens to be called 'subnode' in this particular case :)