AU/ml

 The term "AU/ml" typically stands for "Arbitrary Units per milliliter." 

It's a unit of measurement used in various fields of science, particularly in assays or tests where the exact quantity of a substance is not directly quantifiable in traditional units (e.g., grams, moles, etc.).

The term "arbitrary units" implies that the measurement is based on some arbitrary reference or standard rather than a standardized unit of measurement like grams or moles. 

The specific value of one arbitrary unit doesn't correspond to a specific amount of the substance but rather indicates a relative amount compared to a reference standard or control.

This unit is commonly used in various types of assays where the exact concentration of a substance isn't critical, but rather the relative changes in concentration between samples or conditions are of interest. 

It allows for comparison and quantification of experimental results without necessarily knowing the exact concentration of the substance being measured.