The commonplace, as Richard Lanham tells us:
was a general argument, observation, or description a speaker could memorize for use on any number of possible occasions. So an American statesman who knows he will be asked to speak extempore on the Fourth of July might commit to memory reflections on the bravery of the Founding Fathers, tags from the Declaration of Independence, praise of famous American victories, etc. A few scattered traditional loci death is common to all; time flies; the contemplative vs. the active life; the soldier's career vs. the scholar's; praise of a place as paradisiacal; the uses of the past; a short, celebrated life vs. a long, obscure one.
(Lanham, Richard. Handlist of Rhetorical Terms (University of California Press: Berkeley, 1994).
The American Heritage Dictionary defines a commonplace book as "a personal journal in which quotable passages, literary excerpts, and comments are written."
I've written about the striking similarities between weblogs or blogs and the traditional commonplace book elsewhere. This blog is an experiement in using a blog as my commonplace book, instead of the various notebooks, files, scraps of paper and HyperCard stacks I've used in the past.