A discourse marker is a word or phrase that is relatively syntax-independent and does not change the truth conditional meaning of the sentence, and has a somewhat empty meaning. Examples of discourse markers include the particles "oh", "well", "now", "then", "you know", and "I mean", and the 'discourse connectives', "so", "because", "and", "but", and "or".
In Practical English Usage, Michael Swan defines a 'discourse marker' as 'a word or expression which shows the connection between what is being said and the wider context'. For him, a discourse marker is something that either connects a sentence to what comes before or after, or indicates a speaker's attitude to what he is saying. He gives three examples: on the other hand; frankly; as a matter of fact.. Ian McCormick's The Art of Connection outlines nine classes of connectives based on their purpose:
1. to provide a sense of where something is in relation to something else; 2. to supply a sense of when something is happening; 3. to compare two ideas and express similarities; 4. to contrast ideas English provides many examples to signal the notion of difference; 5. to present additional or supplementary ideas; 6. to indicate that a point in a discussion has been conceded or already taken into account; 7. to demonstrate a sense of logical sequence; 8. to offer an illustration or an example; 9. to deliver a summary of the ideas discussed.
Traditionally, some of the words or phrases that were considered discourse markers were treated as "fillers" or "expletives": words or phrases that had no function at all. Now they are assigned functions in different levels of analysis: topic changes, reformulations, discourse planning, stressing, hedging, or backchanneling. Those functions can be classified into three broad groups: (a) relationships among (parts of) utterances; (b) relationships between the speaker and the message, and (c) relationships between speaker and hearer. An example of the latter is the Yiddish involvement discourse marker nu, also used in Modern Hebrew and other languages, often to convey impatience or to urge the hearer to act (cf. German cognate nun, meaning "now" in the sense of "at the moment being discussed," but contrast Latin etymological cognate nunc, meaning "now" in the sense of "at the moment in which discussion is occurring"; Latin used iam for "at the moment being discussed," and German uses jetzt for "at the moment in which discussion is occurring").
Data shows that discourse markers often come from different word classes, such as adverbs ("well") or prepositional phrases ("in fact"). The process that leads from a free construction to a discourse marker can be traced back through grammaticalisation studies and resources.
- Benjamin Brown, '“But Me No Buts”: The Theological Debate Between the Hasidim and the Mitnagdim in Light of the Discourse-Markers Theory'
- Benjamin Brown, '"Some Say This , Some Say That": Pragmatics and Discourse Markers in Yad Malachi's Interpretation Rules' 
- Carol Lynn, Moder; Aida Martinovic-Zic (2004). Discourse Across Languages and Cultures. John Benjamins Publishing Company. p. 117. 9027230781.
- Schiffrin, Deborah (1986), Discourse markers, Studies in interactional sociolinguistics, 5., Cambridge [Cambridgeshire], ISBN 978-0-521-30385-9
- Swan, Michael (2005). Practical English Usage. Oxford: Oxford University Press. p. xviii. ISBN 0-19-442098-1.
- McCormick, Ian. (2013) The Art of Connection: the Social Life of Sentences. Quibble Academic.
- Zuckermann, Ghil'ad (2009). Hybridity versus Revivability: Multiple Causation, Forms and Patterns. In Journal of Language Contact, Varia 2: 40-67, p. 50.