Lewis & Short

collīsĭo (conl-), ōnis, f. [collido],

  1. I. a dashing or striking together, a concussion, shock: partūs abjecti, Just. 11, 12, 6.
  2. II. Transf., of letters in a word: πνεῖν, χεἰν, ῤεῖν, non sunt integra, sed ex collisione contracta. Macr. Diff. 19, § 17.