What is Agile Software Development?

It's a set of values, principles, and practices for sustainable software development

• at its core, it is a mentality of expecting and embracing change and being able to effectively adapt
• and on a fundamentally related note:
  • code that is Easy To Change is the foundation of a team's ability to be agile

Some key sources:

• Extreme Programming
  • a development methodology seminal to Agile
• Agile Manifesto (2001)
  • the official manifesto along with authors, signers, and a story of how the manifesto came to exist
• Craftsmanship Manifesto (2009)
  • a followup to the Agile manifesto after some years of semantic diffusion
  • it includes emphasizing quality software as a fundamental ingredient
• Agile Software Guide
  • a high-level overview along with collections of sources referenced in relation to each section in the guide--essence of Agile development, technical practices, collaboration, problems (semantic diffusion)

I also appreciate this blog post by Robert Martin--a reminder of the history behind Agile and clarification of what Agile is and is not:

• the blog post
  • Agile was about software, started by programmers
    • it began as XP
      • Extreme Programming, a Reflection
      • Extreme Programming
  • it's about quality software that enables software teams to effectively and sustainably produce value by easily adjusting to changing business requirements
  • it is not about project management