Define Tautology Meaning

Some people think this is an example of tautology:

It would be easy to find a blind man in a nudist colony because it wouldn't be hard.

"Either we'll get in trouble, or we won't" is a logical tautology. By including all possibilities the statement must inherently be true.

In "PIN number" the word "number" is a tautology because a PIN is always a number. (At least that's what the N originally stood for — if the term PIN evolved to include letters someday then PIN number would no longer be a tautology.)

In "morning sunrise" the tautology is "morning" because sunrises are a subset of mornings; removing the first word removes no meaning. (The addition of "morning" may be aesthetically more pleasing, in a poem for example, but it remains a logical tautology.)

Jack built a brick house out of bricks.
The filing cabinet had inertia, it wouldn't budge.
Go sit in the corner where the walls and floor meet, boy!
The tautologous tautology

In a tautology "is" is 'is' (and 'is' is "is").

A tautological tautology.

HIV virus
PIN number

Example of a tautology: Bad people take drugs; therefore, people who take drugs are bad.
The other definition appearing here, "Unnecessary repetition of a word", is a crudification and wrong - such pointless reiterative repetition is a redundancy, and one who makes such a definition is called a "redundunce". Consult Fowler.

What is a tautology?
a tautology.

Examples of tautology:

It was a wet rainy day with lots of precipitation.
She is a female woman of the opposite sex, John.
We could see some dilapidated ruins.

"It's rainy and wet outside" is a tautological statement