Elementary Metabolic Unit
The Elementary Metabolic Units (EMU) are the subset of labelled Isotopomers. Thus for an $n$ atom molecule there exist $2^n-1$ EMUs.
Example: Consider the subset of three atom molecules with at least one atom of the molecule labeled. The subset consists of seven EMUs.
I contrast to other approaches that always require the modelling of all isotopomers (or cumomers) it is possible to work with a subset of EMUs only, thereby the computational costs are significantly reduced.
The number of molecules in an EMU determines its size. In the shown example there are three EMUs of size one and size two respecively. Only one EMU with size three is present.