Question 42, article 1 of the Second Part of the Second Part of the Summa Theologica asks “whether sedition is a special sin”, and Aquinas answers it in the body of the article. The Summa is the standard summa of scholastic theology, and he sets “sedition” beside “war” and “strife”, the vices opposed to peace. It stands under this term as the definition of the word.
Read in contextSedition is a special sin, having something in common with war and strife, and differing somewhat from them. It has something in common with them, in so far as it implies a certain antagonism, and it differs from them in two points. First, because war and strife denote actual aggression on either side, whereas sedition may be said to denote either actual aggression, or the preparation for such aggression. Hence a gloss on 2 Cor. 12:20 says that "seditions are tumults tending to fight," when, to wit, a number of people make preparations with the intention of fighting. Secondly, they differ in that war is, properly speaking, carried on against external foes, being as it were between one people and another, whereas strife is between one individual and another, or between few people on one side and few on the other side, while sedition, in its proper sense, is between mutually dissentient parts of one people, as when one part of the state rises in tumult against another part. Wherefore, since sedition is opposed to a special kind of good, namely the unity and peace of a people, it is a special kind of sin.