What is I Know It When I See It?

Legal Definition
The phrase "I know it when I see it" is a colloquial expression by which a speaker attempts to categorize an observable fact or event, although the category is subjective or lacks clearly defined parameters. The phrase was famously used in 1964 by United States Supreme Court Justice Potter Stewart to describe his threshold test for obscenity in Jacobellis v. Ohio. In explaining why the material at issue in the case was not obscene under the Roth test, and therefore was protected speech that could not be censored, Stewart wrote:

I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description ["hard-core pornography"], and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that.

The expression became one of the most famous phrases in the entire history of the Supreme Court. Though "I know it when I see it" is widely cited as Stewart's test for "obscenity", he never used the word "obscenity" himself in his short concurrence. He only stated that he knows what fits the "shorthand description" of "hard-core pornography" when he sees it.

Stewart's "I know it when I see it" standard was praised as "realistic and gallant" and an example of candor.
-- Wikipedia