What is Expectation Of Privacy?

Legal Definition
Expectation of privacy is a legal test which is crucial in defining the scope of the applicability of the privacy protections of the Fourth Amendment to the United States Constitution. It is related to, but is not the same as, a right to privacy, a much broader concept which is found in many legal systems (see privacy law).
-- Wikipedia
Legal Definition
The expectation of privacy test, originated from Katz v. United States is a key component of Fourth Amendment protects people from warrantless searches of places or seizures of persons or objects, in which they have an subjective expectation of privacy that is deemed reasonable in public norms. The reasonableness standard is construed upon the totality of circumstances on a case-by-case basis. The person’s precautions taken to exclude others’ access are strong indicators to the expectation of privacy and might be taken into consideration by the court.

The legitimate expectation of privacy must have a independent source outside Fourth Amendment. For example, private homes are at the core of Fourth Amendment protection subject to a few exceptions, as they are closely associated with the ownership interest in property law.

On the other hand, a warrantless seizure of abandoned property usually does not violate the Fourth Amendment. Temporary residents on public property without permit and in violation of law do not have legitimate expectation of privacy either. Moreover, the Fourth Amendment protection does not expand to governmental intrusion and information collection conducted upon open field is not considered reasonable. There are some exceptions under state laws that grant protection to open fields.

Relevant reading:

Katz v. United States