Reactionary is not seen as a coherent set of views that anyone professes, but only a boo word from leftists which has generally been superseded by "racist, sexist, homophobe".

The same critique could be made of conservatism. If we accept that term, reactionary seems to be acceptable as well:

A reactionary is an individual that holds political viewpoints which cause them to seek to return to a previous state (the status quo ante) in a society. Reactionaries are considered to be one end of a political spectrum whose opposite pole is perceived radicalism, though reactionary ideologies may be themselves radical. While it has not been generally considered positive to be regarded as a reactionary it has been adopted as a self-description by some such as H. L. Mencken,[1] Gerald Warner of Craigenmaddie[2] and John Lukacs.[3]

In other words reactionary is the right wing person who looks around the society he inhabits and does not see much worth preserving but seeks to revive older institutions. Julian the Apostate seems a good example.

A member of the Constantinian dynasty, he was made Caesar over the western provinces, by Constantius II in 355, where he campaigned successfully against the Alamanni and Franks. Most notable was his crushing victory over the Alamanni in 357 at the Battle of Argentoratum - despite being outnumbered. In 360 in Lutetia (Paris) he was acclaimed Augustus by his soldiers, sparking a civil war between Julian and Constantius. However, Constantius died before the two could face each other in battle, naming Julian as his rightful successor. In 363, Julian embarked on an ambitious campaign against the Sassanid Empire. Though initially successful, Julian was mortally wounded in battle and died shortly after.

Julian was a man of unusually complex character: he was "the military commander, the theosophist, the social reformer, and the man of letters".[4] He was the last non-Christian ruler of the Roman Empire and it was his desire to bring the Empire back to its ancient Roman values in order to save it from dissolution.[5] He purged the top-heavy state bureaucracy and attempted to revive traditional Roman religious practices at the cost of Christianity. His rejection of Christianity in favour of Neoplatonic paganism caused him to be called Julian the Apostate (Ἀποστάτης or Παραβάτης "Transgressor") by the church.[6] He was the last emperor of the Constantinian dynasty, the empire's first Christian dynasty.

Besides causing us to wonder how much better the world might have been had he lived, he provides a clear contrast to the meek conservative Roman senators petitioning for the Altar of Victory be restores to the curia not as an object of worship but of symbolic value.

In 19th and 20th century Europe the word is tied to a particular set of views similarly to how big C Conservatism in the US is liked to certain views not just a small c conservative outlook of careful and slow change. The European Reactionary may support monarchism, pro-clericalism, anti-bourgeois and what Americans might term Paleoconservative and Paleolibertarian positions.

The very similar but secular reactionary cluster of views that I expect is overrepresented on LessWrong is the 21st century internet made Neoreactionary one (here is a leftwing take). Besides Moldbug, read Nick Land and Foseti's links to get a further feel for it.

a different set of ideas that are both Jacobite-ancient and internet-au-courant