You can use full words or even short multi-world phrases in math expressions. From a typesetting POV, write them in roman (upright) type, and enclose them in br

You can use full words or even short multi-world phrases in math expressions. From a typesetting POV, write them in roman (upright) type, and enclose them in brackets whenever ambiguity arises (due to the possibility of separating spaces within a single variable, or the concurrent use of juxtaposition (of single letters) as a product or for other purposes within the same sorts of expression. This is very common in the applied sciences in general, and the convention is universally understood.