Sources: Wikipedia and this rather strange article
Clinamen is the Latin name Lucretius gave to the unpredictable swerve of atoms, in order to defend the atomistic doctrine of Epicurus.
When atoms move straight down through the void (kenon) by their own weight, they deflect a bit in space at a quite uncertain time and in uncertain places, just enough that you could say that their motion has changed. But if they were not in the habit of swerving, they would all fall straight down through the depths of the void, like drops of rain, and no collision would occur, nor would any blow be produced among the atoms. In that case, nature would never have produced anything
Clinamen is the word that Lucretius, in his 2nd century book, The Nature of Things, used to describe how the world works. Today, people are not inclined to read a physics book that is 2000 years old, but it is not the particular beliefs that Lucretius may have held that interest us but, rather, the way he used inversion to uncover a key truth that is just as true in the world of particle colliders and Hibbs’ Fields as ever.
Instead of describing solid things as fixed and resting in space, he got his readers to imagine that the whole solid spatial world was moving along in the same direction. Our analogy would be a 12 lane highway where commuter traffic is all moving along at exactly the same speed. In terms of the cars and trucks on the highway, there is no apparent motion, although they are all traveling, if they are lucky, at 60 mph.
What creates a sensation, however, is when something swerves. He called this a “clinamen,” and we can imagine how velocity — a car shooting forward or one slowing down — creates an issue.