Equilibrium

[David Myer]

I see that this word was discussed in the Goodword of the day in 2005. But the lengthy discourse there doesn't answer my question. I just wrote "We need an equilibrium in the system" and Microsoft Word corrected me (how dare it!) It wants no indefinite article, and reflecting on the suggestion I agree that it is sweeter to write "We need equilibrium in the system". But why can't we have an article (definite or indefinite) in front of it. It is a straightforward enough noun. And it seems to me OK to say "Let us not disturb the equilibrium". Indeed, it would be awkward without the article. And Microsoft accepts this. So assuming Microsoft is working to a rule, what is the rule?

[Dr. Goodword]

Equilibrium is a mass noun, that means it is always singular; there are no equilibriums (or equilibria). An means one of many, which does not apply to mass nouns. They have no plural, therefore they can have no singular. However, singular is the default number for such words, too.

[David Myer]

Thank you. Clear and concise. Very helpful.