Organised by recurring meaning rather than by time or place. Of an ordering that follows what things are about.
Greek θεματικός (thematikos), from θέμα (thema, a thing laid down, a proposition), from τίθημι (tithenai, to place); organising by recurring meaning follows what a thing has been laid down as being about, not where or when it happened.
An arrangement that groups by what things are about rather than by when or where they occurred. Content sorted this way finds its neighbours through shared subject, never through a shared date.