字面意思是任意单位。wiki上的解释，大概是描述一种相对关系。通常会有一个参照对象，然后定性地描述想要测量的对象与参照对象的相对变化趋势。因为只是描述一种比值关系，所以可以视为无量纲的，或者说可以用任意单位描述。

In science and technologyan arbitrary unit (abbreviated arb. unit,[1] see below) or procedure defined unit[2] (p.d.u.) is a relative unit of measurement to show the ratio of amount of substanceintensityor other quantitiesto a predetermined reference measurement. The reference measurement is typically defined by the local laboratories or dependent on individual measurement apparatus. It is therefore impossible to compare "1 arb. unit" by one measurer and "1000 arb. unit" by another measurer without detailed prior knowledge on how the respective "arbitrary units" were defined; thusthe unit is sometimes called an unknown unit.[2] The unit only serves as a placeholder unit to compare multiple measurements performed in similar environment.
Since the unit expresses the ratio between the measurement and the referenceit can be considered a unit for dimensionless quantities. The unit is commonly used in fields such as physiology to indicate substance concentrationand spectroscopy to express spectral intensity.