(Latin: "decree of the Senate")
In ancient Rome, the Senate had the power to issue summary decrees with the force of law. Usually, these were applied to rush legislation for urgent matters, such as finances, etc., although they could also apply to general issues of the safety of the state (such as the Senatus consultum de bacchanalibus issued in 186 BCE, instituting widespread prohibitions on various forms of worship of Bacchus).
On rare occasions, the senatus consultum ultimum ("final decree of the Senate") was issued. This was effectively a decree instituting martial law and placing all civil and military authority in the hands of a single person, usually a consul. The decree was in effect for the duration of the crisis which occasioned its issue.
Though the intent of the senatus consultum ultimum was to preserve the Republic in the face of unusual and extreme circumstances, it was often abused. As the Republic waned, in the last century BCE, the s.c.u. was used more and more frequently, and for more blatantly partisan purposes each time.
In the long run, this habit of issuing summary powers to individuals proved to be the undoing of the Senate's own power base.