"A specific series of representations, practices and performances through which meanings are produced, connected into networks, and legitimized." * 

Discourses attempt to 'naturalize' and often universalize a particular world view or narrative that places agents, such as people, places, acts and non-humans etc., within specific hierarchies and power relationships. The discourses and narratives relating to the 'War on Terror' or the 'Axis of Evil' are oft used (if now clichéd) examples of this.   

No single definition is capable of encapsulating the entirety of what is meant by 'Discourse', with many competing and subtlety distinct meanings vying with one another within different areas of the social sciences.

However, the Dictionary of Human Geography* identifies 5 characterisations that can be derived from the above definition, although these are in no way comprehensive:

 

 Discourse as heterogeneous: discourses are not fixed or the product of an individual text, archive or statement, but rather are 'built up' or altered and carry multiple meanings and implications whilst also moving through different domains and registers. For example, what is understood to be 'natural' is not defined in a single place, but is rather a complex and possibly confused amalgamation of numerous inputs and dependent of upon time, place, circumstance, and individual or group and so on. 

Discourse as regulated: discourses are coherent and systematic, despite being in competition with one another and often contradictory in nature. The borders between discourses are marked by "regimes of truth" and seek to determine what can be included and what be excluded in addition to attempting to establish a criteria of what is acceptable. For example, elements of the Christian right often seek to critizise homosexuality by trying to define a specific and bigoted interpretation of what is 'natural', in addition to placing certain information, such as that found in the Bible, as being 'more valid' than texts that arguably give a different viewpoint. 

 Discourse as embedded: discourses are "materially implicated in the conduct of social life", or 'embedded', and are therefore not purely theoretical constructions but rather can be seen as having a physical impact that is created by, and imprinted upon, within, through and across institutions, peoples, places, non-humans and power relationships that exist between them. Society and discourse therefore act to co-produce one another, and the outcome can be seen in the physical world around us and the agents within it. For example, the airport could be seen as a physical manifestation of multiple discourses (such as security, health, imagined geopolitical hierachies, class, wealth, race etc) that dictate who and what can be moved where and for what reasons.

 Discourse as situated:  discourses inevitably provide partial knowledges that are situated within the contexts in which they exist over time, and so cannot be seen as closed, immutable or complete. Rather, they are "characterised by particular constellations of power and knowledge and are always open to contestations and negotiation". Essentially, the meanings that areproduced, connected into networks, and legitimized will vary depending on the context. See also Actor Network Theory.

Discourse as performative: discourse in fact co-produces "the social" and "the self", rather than just describing it - at some level, they create the realities that they purport only to describe.  For example, Colin Powell's now infamous presentation to the UN regarding the existence of weapons of mass desturction in Iraq acted to perform a discourse that was (at the time) capable of justifying pre-emptive invasion and provide a 'lense' through which to view Iraq, its governement and its people, through the use of speach, imagery, and 'matters of fact' etc.   

*Johnston, R.J., Gregory, D., Pratt, G., Watts, M. (Eds.) 2006 Dictionary of Human Geography 4th Ed. Blackwell Publishing,Oxford.