except that word, then as a general rule the dictionary
will help to make the meaning clear. But when the difficulty is caused,
not by a word being unfamiliar, but by its being used in a certain
context, then the best dictionary in the world is, for your purpose, of
no use at all. The nature of every dictionary is necessarily such that
it entirely leaves out of account all doubts about meaning which are of
this second kind. The most that a dictionary can do is to tell us the
meaning of a word in those cases where the context in which it is used
is _not_ such as to make the meaning doubtful.[8]

In practice the words which most often need definition are those which
are, as it were, shorthand symbols for perhaps a very extensive meaning.
Unless the limits of this extended meaning are clearly marked out you
cannot tell whether the minds of your readers are, as the lawyers say,
running on all fours with your own or not. This extended meaning may be
of various sorts: for example, it may be a large general principle, as
in the case of "evolution" or "culture"; or it may be a general system
or practice, as in the case of "commission government," "honor system,"
or "high standards for graduation"; or it may be a general class of
things, persons, or events, as in the case of "secondary school,"
"professional coach," or "murder." When you use any such term in an
argument, it is essential that your readers shall have the same set of
details, ramifications, or instances in mind as you have yourself. For
this purpose you must define the term; or, in other words, you must lay
out or display the ramifications and limitations of the principle, the
details of the system or practice, or the exact kinds of things,
persons, or events, which you have in mind when you use the term. A few
examples will make this practical meaning of defining clear.

Sometimes the definition proceeds by careful and specific limitation of
the general signification of a word, as in the following example from
Bagchot:

I should say that except where it is explained to the contrary, I
use the word "toleration" to mean toleration by law. Toleration by
society of matters not subject to legal penalty is a kindred
subject, on which if I have room I will add a few words; but in the
main I propose to deal with the simpler subject, toleration by law.
And by toleration, too, I mean, when it is not otherwise said,
toleration in the public expr