Greek παρέγκλισις (parenklisis); Latin clinamen (Lucretius)
A minimal, unpredictable deviation of atoms from their straight path. Our main sources for it are Lucretius, who calls it the clinamen, and Cicero; the surviving letters of Epicurus do not mention it. The swerve explains how atoms come to collide at all, and it breaks the chain of necessity that would otherwise rule out free will. See There Is No Necessity To Live Under the Control of Necessity.
1. Key Texts
On this topic, another thing I want you to know is this. When bodies are being borne by their own weight straight down through the void, at quite uncertain times and places they veer a little from their course, just enough to be called a change of motion. If they did not have this tendency to swerve, everything would be falling downward like raindrops through the depths of the void, and collisions and impacts among the primary bodies would not have arisen, with the result that nature would never have created anything. But if anyone should suppose it possible that heavier bodies, by virtue of their faster travel in a straight line through the void, should land from above on lighter ones and in that way produce impacts capable of leading to creative motions, he is departing far from true reasoning. For whatever things fall through water and thin air must necessarily fall at a speed proportionate to their weight, since the body of water and the rarefied substance of air cannot impede everything equally, but succumb and yield faster to heavier things. But the emptiness of void, by contrast, can nowhere and at no rime stand up to anything, but must constantly, as its nature demands, give way. Through the passive void, consequently, ail things must travel at equal speed, regardless of differences in weight. Hence it will never be possible for the heavier ones to land from above on the lighter ones and by themselves produce impacts resulting in the variety of motions through which nature does her work. Thus it follows once again that the bodies must veer off a little - no more than one minimum, lest we item to be inventing oblique motions and be refuted by the facts. For we see as plain and obvious that weights when hurtling downwards have no intrinsic power to travel obliquely. But who in the world can see that they do not swerve from their straight trajectory at all? (tr. Long & Sedley)