Definition of centillionth in US English:

centillionth

noun

rare
  • Any of the centillion equal parts into which something may be divided; (hyperbolically) an infinitesimal fraction.

adjective

rare
  • That is one of the centillion equal parts into which something may be divided; (hyperbolically) infinitesimally small.

Origin

Early 19th century; earliest use found in Annals of Philosophy.