The Merriam-Webster Dictionary defines bilingualism as the ability to speak two languages. Now, that is a really specific definition. For that matter, how well do you need to speak both languages? How many words you need to have in your vocabulary? How often do you need to speak each language? If you can read both, are you bilingual? What about if you understand the spoken words but don’t speak it? What if you can write both? Where is the fine line when you stop being a monolingual and start being a bilingual?
I believe that your are bilingual, to a degree, when you can function and can fulfill your needs requiring the two languages. I don’t think there is a fine line that you cross and become bilingual. I propose there is a zone in which you start to be bilingual to a degree. Once you progress further enough inside the zone, the you become fully bilingual. The definition presented is then really defining full bilingualism.