I recently started reading a book by George Saunders called Bilingual Children: From Birth to Teens. The book is full of wonderfully described terms in definitions quite easy to understand by the non-specialist reader. Saunders writes about his family including himself, wife, and three children. The children were raised speaking German and English. He shares their experiences along with other findings of his own and others' research on bilingualism.
"Bilingual families are by no means a rarity in today's world. Indeed, bilingualism is a far more common phenomenon than the predominantly monolingual native speakers of a world language such as English might realize. In fact, bilinguals outnumber monolinguals" (Saunders, 1988, pg. 1). Saunders addresses the fact that there are various levels or degrees of bilingualism and that the word holds different connotations for different people.
A person's level of language can be assessed by the four skills of listening comprehension, speaking, reading comprehension, and writing. The degree of bilingualism would be determined by assessing these skills in both language.
I am by no means a bilingual who could pass as a native speaker, but I do hold a degree of bilingualism.
French, we have a long journey ahead of us. I am extremely jealous of my daughters who have been exposed to two languages since birth. Learning a second language as an adult is proving a challenge for me.