Language: a variety, or code, that is geo-politically defined as belonging to a specific nation and that functions as the common vehicle of communication across heteronymous dialect varieties.
Dialect: a regional and/or a social variety that is shared by a group of speakers in a common language-cultural area that is related but different from the standard language on a lexical, grammatical and phonological level. In the absence of a common standard language, dialect comprises the lexically, grammatically, and phonologically distinct but related code between mostly neighboring groups of speakers (example: the dialects of ancient Greek).
Accent: is a social and /or regional distinct way of pronunciation, or it can emerge as a by-product of language acquisition.
Variety: a form of language that is shared by a certain group of speakers which is distinguishably different from other related groups of speakers.
Lingua franca: is the common language used by people of different language background to communicate with each other. Koiné can be a synonym of lingua franca.
Pidgin: contact language that arises when speakers of different language backgrounds need to communicate with each other for restricted purposes, for instance: trade or slavery (this process is called cross-linguistic negotiation). Often there is a substratum and a superstratum relation between the languages involved.
Creole: like pidgins, creoles develop in contact situations that typically involve more than two languages. In contrast to a pidgin, a creole is the native language of a speech community. When a pidgin extends its functional range, becomes a daily means of communication and is spoken as an l1/l2 by a new generation of speakers pidgin is turned into creole.
Linguicism (Phillipson 1988, 1997): the ideologies and structures which are used to legitimate, effectuate and reproduce an unequal division of power and resources between groups which are defined on the basis of their language.