Exposition = Explanation.
Oxford English Dictionary Exposition. 3.a. The action of putting out to public view; an instance of this; a display, show, exposure
4.a. The action or process of setting forth, declaring, or describing, either in speech or writing.
5.a. The action of expounding or explaining; interpretation, explanation. Also an instance or mode of this; an explanation, interpretation.
In drama, [exposition is] the presentation of essential information regarding what has occurred prior to the beginning of the play. (www.tnellen.com/cybereng/lit_terms/terms/Literary.Terms.html)
The immediate revelation to the audience of the setting and other background information necessary for understanding the plot; also, explanation. (kmhs.typepad.com/parrott_ap_english_langua/files/rhetorical_terms.doc)
The part of a play that provides the background information needed to understand the characters and the action. (http://www.answers.com/topic/exposition#)
Expository dialogue is characters' speech that provides important explanatory information and contexts for actions, decisions, backgrounds, etc.
Writing intended to explain the nature of an idea, thing, or theme. Expository writing is often combined with description, narration, or argument. (www.novelguide.com/a/discover/nfs_0000_0023_0/nfs_0000_0023_0_00026.html)
Writing that explains. (www.butte.edu/departments/cas/tipsheets/style_purpose_strategy/definitions.htm)