Rule by decree is a style of governance allowing quick, unchallenged promulgation of law by a single person or group of people, usually without legislative approval. While intended to allow rapid responses to a crisis, rule by decree is easily abused and is often a key feature of dictatorships.
When a state of emergency, such as martial law, is in place, rule by decree is common. While rule by decree is easily susceptible to the whims and corruption of the person in power, it is also highly efficient: a law can take weeks or months to pass in a legislature, but can be edited quickly during ruling by decree. This is what makes it valuable in emergency situations. Thus, it is allowed by many constitutions, including the French, Argentine, Indian and Hungarian constitutions.
The expression is also sometimes used when describing actions of democratic governments that are perceived to unduly bypass scrutiny from the legislature or populace.