An imperial decree was an order issued personally by the emperor.
Tips
history
敕 is not used independently in modern Chinese. It originally meant to admonish, then specialized to an emperor's written order; it survives in historical compounds like 敕令 (an imperial edict) and 敕封 (to confer a title by decree).
register
Historical and literary; the modern word is 圣旨 or 命令.