Aiding a nation's declared enemy (especially during wartime), rebellion, insurrection, advocating overthrow of a [legal] government, and misprision of treason (e.g., failure to report treason) are typical examples of acts of treason. You may want to check the actual wording of law on the subject in the U.S.C.A. Title 18 (Crimes), Chapter 115.

Yes. It is a violation of criminal law.

A misprision of a felony is the concealment of a felony from the authorities, but without actually giving any degree of active support to its commission (i.e.: the person is not considered an accomplice to the crime).

For example: Person 'A' robs a bank and kills a teller. He goes to Person'B's home and asks to hide out and tells Person 'B' that they just robbed bank. Person 'B' does nothing, but continues to hide the bank robber anyway. Person'B' has committed a Misprision of a Felony.

in the UK a crime against the crown is called treason

If it opccurred after your 18th birthday it is a permanent part of your criminal history record.