Prior to leaving office (Presumably in 2029)

This will be resolved based on whenever the House of Representatives adopts a resolution impeaching Donald J. Trump. This market will close as soon as the House adopts such a resolution, and it will resolve based on the date of adoption as soon as the resolution is signed. If a motion to reconsider, rescind, or vacate the resolution is introduced before the resolution is signed the market will reopen. 

Trump will be deemed to still be in office at the time of impeachment if the resolution, when adopted by the House, uses language such as

That Donald John Trump, President of the United States, is impeached for high crimes and misdemeanors" 

If language indicates that they are impeaching a former president, then he will be deemed not to be in office. 

If a resolution of impeachment and articles of impeachment are adopted separately, the time of the impeachment resolution's adoption will control.

It will be assumed, absent evidence to the contrary, that a president cannot be impeached after death. Should the president die, the market will resolve if, after two weeks of the president's death, there is no talk of impeaching the president posthumously. The president will not be considered to have left office if section 4 of the 25th Amendment is invoked.