Is such a thing even defined under the Constitution? It never was questioned at the time. That doesn’t mean that, say, Donald Trump can’t come along and question it.
In 1866, the Supreme Court ruled in Ex parte Garland that the pardon power "extends to every offence known to the law, and may be exercised at any time after its commission, either before legal proceedings are taken, or during their pendency, or after conviction and judgment." (In that case, a former Confederate senator successfully petitioned the court to uphold a pardon that prevented him from being disbarred.) Generally speaking, once an act has been committed, the president can issue a pardon at any time—regardless of whether charges have even been filed.