Dialect:

Technically, any variety of language spoken by a particular group of people. However, the most common use is for a geographical variety of a language.

Dialect continuum:

When the regional varieties of a language form a continuum of small, incremental changes where neighbouring varieties are always fully mutually intelligible, but intelligibility reduces with distance. Western Romance (Portuguese to Southern Italian) used to form a dialect continuum, as did German-Dutch and the Iroquoian languages of the original Five Nations. The main reason why the question “language or dialect” is so hard to answer.

Dialect levelling:

The process of reduction of the variety in one or several dialects, usually through contact with one or more other dialects, most commonly a standard language.

Regiolect:

More technical term for a regional variety of a language. May be employed in languages with strong dialectal variation to differentiate the “full” dialect from a regionally-coloured form of the standard language. This could f. ex. be used for Scots (dialect, if not even considered a separate language) vs. Standard Scottish English (regiolect), but doesn’t otherwise really apply to English.

Sociolect:

A language variety spoken by a social group, most commonly based on class or age. Things like youth speak, thieves’ cant, or internet jargon are all forms of sociolect.

Slang:

General word for informal vocabulary; the English term refers to the entire spectrum from informal words that are however common in everyday conversation to words considered vulgar, bordering on the taboo. Also includes in-group speech.

Cant, argot:

Mostly vocabulary-based language variety used by a group with the express aim of excluding outsiders. Most common in forms of thieves’ cant.

Jargon, argot:

Specialised vocabulary from a particular field, occupation, hobby etc. Jargon need not be informal. Note: The distinctions between cant, argot and jargon are not rigidly adhered to.

Register:

Variety of language used for a particular purpose or situation. “Low registers” refers to informal, “high” to formal registers.

Ethnolect:

Language variety spoken by an ethnic group, f. ex. AAVE in English. May overlap with sociolect if the ethnicity is associated with a particular class.

Multiethnolect:

Ethnolect used by several different ethnic groups, typically minorities. Usually found in large metropolitan areas.

Idiolect:

Every individual’s unique use of language.

Code-switching:

A speaker’s ability to switch between dialects (in the larger sense) and/or registers.

Accent:

Way of pronouncing a language by an individual. Most commonly used for either regional or foreign accent, but even the standard pronunciation of a language is an accent (the standard accent).