Here are some definitions of Social Media by different sources online.

Social media are primarily Internet-based tools for sharing and discussing information among human beings. …
http://en.wikipedia.org/wiki/Social_media

A category of sites that is based on user participation and user-generated content. They include social networking sites like LinkedIn, Facebook, or My Space, social bookmarking sites like Del.icio.us, social news sites like Digg or Simpy, and other sites that are centered on user interaction.
http://www.lazworld.com/glossary.html

Online technologies and practices that people use to share opinions, insights, experiences, and perspectives with each other.
http://www.tvb.org/multiplatform/Multiplatform_Glossary.aspx

Social media are works of user-created video, audio, text or multimedia that are published and shared in a social environment, such as a blog, wiki or video hosting site.
http://www.capilanou.ca/help/active-cms/glossary.html

An umbrella term that defines the various activities that integrate technology, social interaction, and the construction of words and pictures. …
http://www.anvilmediainc.com/search-engine-marketing-glossary.html

Software tools that allow groups to generate content and engage in peer-to-peer conversations and exchange of content (examples are YouTube, Flickr, Facebook, MySpace etc)
http://www.bottlepr.co.uk/glossary.html