What's a compendium?
If you asked me before today, I would have said "A big gathering of stuff, like an anthology or something." Perhaps I would have phrased it more eloquently.
The word doesn't mean that, however.
A compendium is actually a summary or outline of some substantive work. Or, in the words of oh-so-definitive Merriam-Webster:
1 : a brief summary of a larger work or of a field of knowledge : ABSTRACT
2 a : a list of a number of items b : COLLECTION, COMPILATION
As you notice, the meaning I mention is in there, but only at the very end. That's also known as "the place where dictionaries put the wrong meanings that unknowing folks have perpetuated."
My thanks for this tidbit go to Theodore M. Bernstein's "The Careful Writer." He theorized that the incorrect meaning took hold because the word sounds so lumbering and immense. It's an impressive-seeming word.
Too bad the language doesn't bear it out.