Priming is the process by which a given stimuli activate mental pathways, thereby enhancing the ability to process subsequent stimuli related to the priming stimuli in some way. The proces results in a priming effect, which is the condition where access to a particular item of information in memory is enhanced as a result of recent exposure to a related stimulus. For example, after having viewed a documentary on flight controlling in TV, the next day you may find yourself bringing up the "controller software engineering pattern" in a discussion with your collegues at work. The documentary on flight controlling had primed you in that the memory traces of the concept of a "controlling activity" were highly accesible.
Priming affects all aspects of our behaviour. For example, Syntactic Priming is the tendency to use syntactic structures that parallel the structures of sentences we have heard recently: A speaker who has just heard a sentence in passive voice is subsequently more likely to use a passive construction himself. Our language is in other words a function of the language we hear ourselves and while this is hardly surprising, it is still interesting that priming is responsible for this effect to a great extent. Taking this a bit further, the use of language in turn has an influence on concept formation, which is what the discipline psycholinguistics is concerned with.
Relevance for interaction design
When designing artifacts, computer systems etc. close attention should be payed to the rhetoric we design into the system since the rhetoric will influence the user's understanding of the task domain or context of use. For example, in making a computer system for a dental practice, it makes a difference whether you call the patrons "customers" or "patients". For at secretary working with the system every day, the rhetoric may have a rub-off effect on her, inciting her to view the dental practice's patrons from either a business perspective (customers) or from a care-giving perspective (patients). Similarly, if we as designers talk about learning as "aquiring information" or "communicating knowledge of the new system to the users", this rhetoric does not underscore the constructionistic nature of learning and may influence the way we understand the concept of learning.
The concept of priming can also be applied more directly to interface design. Using priming you are able to "key a user in on" a certain way of thinking, or predispose (i.e. priming) a user to make certain choices by the rhetoric you use.
Priming apparently also has affective dimensions. Zajone 1980 performed an experiment where he showed a list of chinese characters to his test subjects. He subsequently showed the subjects a list of chinese words and asked them which words they "liked the best". The words containing characters presented in the first list, were evaluated more positively.