There is a wide range of synonyms for the word "ten-thousandth" that can be used to describe an infinitesimally small portion of something. Some of the most common synonyms for this term include "one in ten thousand," "0.0001," "100 ppm (parts per million)," or "0.01 percent." Other synonyms might include "rare," "scant," "tiny," or "miniscule." These words all indicate a very small amount or fraction of something, and can be used to describe everything from chemical concentrations to the likelihood of events occurring. Whatever your needs, there are plenty of words to choose from when describing the tiniest fractions of things.