1) Diglottism 
• (n.) Quality of being bilingual.
(from the article `language`) ...are learned to different degrees of competence under various conditions, but the majority of the world`s population remains largely monolingual. ... The learning of a second and of any subsequently acquired language is quite a different matter. Except for one form of bilingualism (see below), it ... ....
Bi·lin'gual·ism noun Quality of being bilingual. « The bilingualism of King's English. Earle. » 
Bilingualism is the ability to use two languages. However, defining bilingualism can be problematic since there may be variation in proficiency across the four language dimensions (listening, speaking, reading and writing) and differences in proficiency between the two languages. People may become bilingual either by acquiring two languages at the ...
Type: Term Pronunciation: bī-ling′gwăl-izm Definitions: 1. Use of two languages in which neither may be regarded as necessarily secondary to the other. 2. Any fluent use of a second language.
varying definitions going from perfect command of two languages to the ability to use another language for practical purposes, however trivial the use. See second language, ADDITIVE/ SUBTRACTIVE
bilingualism 1. The ability to speak two languages. 2. The habitual use of two languages colloquially.
noun the ability to speak two languages colloquially
the ability to speak two languages fluently. · the habitual use of two languages. · a government-supported program encouraging all citizens to acquire a knowledge of both English and French.
The ability to speak two languages with native-like competence. In every individual case one language will be dominant. Lay people often use the term if someone can simply speak a second language well.