what is it called (the offense or charge) when you open someone else's mail illegally because it is not yours

Answer 1

Technically, it would be 3nd degree Mail Fraud (Yes, tampering is the commonly used word). Very close to a larceny (shoplifting). Serious mail fraud is perpetrated from a sender who tries to trick people into sending money or giving out personal information. Tampering or non-violent interception of mail, I believe, used to be classified as robbery; robbery, however, was ruled too serious a charge by the US Supreme Court as it involves sometimes violent confrontation.

Answer 2

Tampering

Answer 3

Since our mail is handles by the U.S. Post office I belive tampering with it is a felony. I don't know the exact technical name.