The relative activity of a species , denoted , is defined as:
where  is the (molar) chemical potential of the species  under the conditions of interest,  is the (molar) chemical potential of that species under some defined set of standard conditions,  is the gas constant,  is the thermodynamic temperature and  is the exponential constant.
Alternatively, this equation can be written as:
In general, the activity depends on any factor that alters the chemical potential. Such factors may include: concentration, temperature, pressure, interactions between chemical species, electric fields, etc. Depending on the circumstances, some of these factors, in particular concentration and interactions, may be more important than others.
The activity depends on the choice of standard state such that changing the standard state will also change the activity. This means that activity is a relative term that describes how "active" a compound is compared to when it is under the standard state conditions. In principle, the choice of standard state is arbitrary; however, it is often chosen out of mathematical or experimental convenience. Alternatively, it is also possible to define an "absolute activity" (i.e., the fugacity in statistical mechanics), , which is written as:
Note that this definition corresponds to setting as standard state the solution of , if the latter exists.