Clinamen (pronounced /klaɪˈneɪmɛn/, plural clinamina, derived from clīnāre, to incline) is the Latin name Lucretius gave to the unpredictable swerve of atoms, in order to defend the atomistic doctrine of Epicurus.
According to Lucretius, the unpredictable swerve occurs "at no fixed place or time":
When atoms move straight down through the void 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.
This indeterminacy, according to Lucretius, provides the "free will which living things throughout the world have."
In English it implies that one is inclined or biased towards introducing a plausible but unprovable clinamen when a specific mechanism can not be found to refute a credible argument against one's hypothesis or theory. Lucretius never gives the primary cause of the deflections. The OED gives its first recorded use in English by Jonathan Swift in his 1706 Tale of Tub ix.166 where he ridicules an unsubstantiated argument:
The Round and the Square, would by certain Clinamina, unite in the Notions of Atoms and Void.
Modern usage 
The OED defines clinamen as an inclination or a bias.
The term has been taken up by Harold Bloom to describe the inclinations of writers to "swerve" from the influence of their predecessors; it is the first of his "Ratios of Revision" as described in The Anxiety of Influence.
In Finnegans Wake, Joyce alludes to the term on the very first words of his work: riverrun, past Eve and Adams, from swerve of shore to bend of bay, brings us by a commodius vicus of recirculation back to Howth, Castle and Environs. If "Eve and Adam's" refers to "even atoms" in the Epicurean sense, the word swerve has a special meaning.
In Difference and Repetition, Gilles Deleuze employs the term in his description of multiplicities, pointing to the observation at the heart of the theory of clinamen that "it is indeed essential that atoms be related to other atoms." Though atoms affected by clinamen engage each other in a relationship of reciprocal supposition, Deleuze rejects this version of multiplicity, both because the atoms are too independent, and because the multiplicity is "spatio-temporal" rather than internal.
Simone de Beauvoir, Jacques Lacan, Harold Bloom, Jacques Derrida, Jean-Luc Nancy, Alain Badiou as well as Michel Serres have made extensive use of the idea of the clinamen, albeit with very different readings.
Lucretius' concept is central to the book, The Swerve: How the World Became Modern.
- Lucretius, ii. 216-224. Translation from Brad Inwood, L. P. Gerson, (1994), The Epicurus Reader, page 66. Hackett
- Lucretius, ii. 251
- Gilles Deleuze, Paul Patton, (1994), Difference and repetition, page 184
- in "The Ethics of Ambiguity" (1948), trans. Bernard Frechtman; Publisher: Citadel Press, ISBN 0-8065-0160-X
- in "The four fundamental concepts of psycho-analysis" (1973), Publisher: W.W. Norton & Co. (April 17, 1998), ISBN 0-393-31775-7
- in "The Anxiety of Influence: A Theory of Poetry" (1973), Publisher: Oxford University Press, USA; 2 edition (April 10, 1997) ISBN 0-19-511221-0
- in "Theory of the Subject" (1982), trans. Bruno Bosteels; (New York: Continuum, 2009): ISBN 978-0-8264-9673-7 (hardcover)
- Hanjo Berressem in Abbas, N. (2005), Mapping Michel Serres, page 53 University of Michigan Press
- Stanford Encyclopedia of Philosophy, Lucretius.