- Expression (language), a thought communicated by language
- Expression (programming), an instruction to execute something that will return a value
- Regular expression, a means of matching strings of text in computing. Think Find-and-Replace.
- Expression (sign language), the expressions and postures of the face and body that contribute to the formation of words when signing
The definitions above are all from the Expression disambiguation page on Wikipedia. There are other ones there too, but I selected these because they strike me as being things that librarians will be quite familiar with, and that are used for conveying information.
But one that is not listed on that page (perhaps I ought to edit it) is:
“Expression is "the specific intellectual or artistic form that a work takes each time it is 'realized.'" An expression of Beethoven's Ninth might be each draft of the musical score he writes down (not the paper itself, but the music thereby expressed) “ – Wikipedia article. Functional Requirements for Bibliographic Records. (viewed 5 June 2013)
It looks a little bit like this..