A dialect is a variety of a language spoken by a group of people that is characterized by systematic differences from other varieties in structure or lexicon. Systematic phonological variation is called accent . This may be due to first language sound substitution as well. Idiolect refers to the individual variation in one person's language use. A group of people speaking the same dialect is called a speech community , and these communities may be defined by a variety of extralinguistic factors, including age, socioeconomic status, and region.
Pure dialects are the result of communicative isolation.
When a speech community becomes isolated from its larger community, its language begins to change and become more and more different from the mother language. Mutual comprehensibility is the key term when defining language versus dialect. When speakers cannot understand each other, linguists consider them to be speaking two languages. If the speakers can understand each other, they are speaking dialects. It must be noted that there is no firm demarcation between dialect and language, rather a continuum.