Discussion Groups / English Usage / January 2010

What is a tautology?

Yes, I know I can look it up, but my question goes beyond what the
dictionary says.

Decades ago I looked it up and the dictionary said "A clear and
obvious truth".  Those 5 words with nothing else iirc.

Now that seems to almost meet the logical definitions below, but not
quite. It doesn't meet the requirement below that the statment be a
compound form, such as "A or not A". How did this happen to me?  Does
anyone remember the definition I remember.

Tautology
    1. needless repetition of an idea, esp. in words other than
those of the immediate context, without imparting additional force or
clearness, as in “widow woman.”
2.     an instance of such repetition.
3.     Logic.
a.     a compound propositional form all of whose instances are true,
as “A or not A.”
b.     an instance of such a form, as “This candidate will win or
will not win.”
RHD

3. 1.       Needless repetition of the same sense in different words;
redundancy.
  2.      An instance of such repetition.
4.   Logic An empty or vacuous statement composed of simpler
statements in a fashion that makes it logically true whether the
simpler statements are factually true or false; for example, the
statement Either it will rain tomorrow or it will not rain tomorrow.
AHD4