Tabular content is anything that makes sense to be in a table, because it has a semantic meaning.
For example the latest posts widget here at the top of the page is tabular content: it is arranged with several headings, and several rows of content where each row is one logical item (a thread) with the relevant attributes for that item displayed under each heading.
Basically, it’s the semantic concept actually applied because tags do actually mean things, not just how people abuse them.
If the content you’re presenting is “a list of things where you have rows and headings and one could cross-reference a column heading and a row item to look something up… it’s a table.”