What does 'cis' mean?

• "http://en.wikipedia.org/wiki/Volkmar_Sigusch"
• http://www.juliaserano.com/whippinggirl.html
• http://alyx.io/educational-resources/trans/
• https://github.com/catamorphism/whatdoescismean
• http://catamorphism.org/
• https://www.patreon.com/mappings