任意单位英文缩写为arb. unit，也称为进程定义单位（缩写p.d.u.） 是在科学或技术领域的一种相对单位，来表示物质的量、强度或其他物理量和一个预定义的参考物理量之间的关系。参考用的物理量一般是由某一实验室订定，或是和个别实验量测的结果有关，因此在不知道参考物理量是否相同的情形下，其即使是针对相同物理量（例如物质的量）定义的任意单位，也无法将一个量测者量到的1任意单位和另一量测者量到的1000任意单位相比较，因此任意单位有时也称为未知单位。此单位只适合量测在类似环境下的多次量测，因为即使参考物理量的数值不明，但量测值和参考物理量之间的比例仍为一无量纲。像物质的量及特殊强度的光谱就常用任意单位来表示。

In science and technology, an arbitrary unit (abbreviated arb. unit, see below) or procedure defined unit (p.d.u.) is a relative unit of measurement to show the ratio of amount of substance, intensity, or other quantities, to 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; thus, the unit is sometimes called an unknown unit. The unit only serves to compare multiple measurements performed in similar environment, since the ratio between the measurement and the reference is a dimensionless quantity even if value of the reference is unknown in conventional units. Units of such kind are commonly used in fields such as physiology to indicate substance concentration, and spectroscopy to express spectral intensity.