Constitutionally, impeachment becomes official when the House sends word of that impeachment to the Senate, triggering a Senate trial.
Impeachment was originally understood to take place when someone from the House formally impeached the president “at the bar of the Senate,” which meant a member of the House formally stated to the Senate that the president (or judge, or other officer) was impeached. That practice lasted from the late middle ages until 1912. Since then, the House has instead sent a written message to the Senate stating that the House “has impeached” the defendant, a message that triggers the trial procedures in the Senate.
Both versions, old and new, depend on the House officially communicating the fact of impeachment to the Senate. That communication has always taken place in short order after the House voted to impeach. The reason lies in the core element of what impeachment is by its very nature: a prosecution by the House that takes place before the Senate. If the message is not sent and the trial is not prosecuted, there is no genuine impeachment in the constitutional sense of the term.
