I assume that you are talking about the test that Harlan proposed in Katz v. United States for when a person has the right to privacy.
If so, Harlan proposed a two-pronged test. First, he said that the person has to act as if they expect that they are in private. In other words, if someone puts something out right in front of their window where everyone can see it, they are not acting as if they are in private. Second, he said that this expectation has to be one that society is willing to recognize as valid. For example, a person having a conversation in the middle of a crowd could not reasonably expect that the conversation would be private.
This was Harlan's privacy test, which was later adopted by the whole Court.
Yes I'm talking about Katz v. U.S.
Why did the majority of the Court reject the trespass doctrine for this case?