Definition:  

Word processing applications offer a range of features, from helping you to focus on the writing task to using virtual notecards.

Tools:

• Mellel: "word processor for Mac OS X designed especially for scholars, creative and technical writers" (Commercial, Mac)
• Open Office: "the leading open-source office software suite for word processing, spreadsheets, presentations, graphics, databases and more" (Free, cross-platform)
• Scrivener: integrated writing platform for novelists, scholars, and others who want to manage the writing process seamlessly (Commercial, Mac)
• WriteRoom: full-screen word processing environment promotes focus on the writing task (Commercial, Mac)

Resources:

See also: 

• Collaborative Authoring