Q. What is a medium of publication?

It sounds like you need to cite something. An example of medium of publication is the instrument by which a work is formatted - it isn't just the written word, but could be any kind of media. It can be print books, websites, electronic books, articles, databases, broadcast, sound recordings, a ballet performance, etc.    

The linked guides below MLA, APA, and Cite the Easy Way show how to create citations for various formats.    
Double check with the style you are required to use for your classes so you'll know how to properly cite your sources.