If you are here to learn what agile means, let me simplify things for you. Online dictionaries say:
agile · adj. 1. able to move quickly and easily.
But if you are looking for the meaning of Agile, there are some things you need to know first.
The "Agile" concept comes from the commonly called "Agile Manifesto." Now, this manifesto is the output of a group of people that met back in 2001. The main topic was what each of them valued about software development at that time. They shared and discussed ideas, practices, different ways of doing stuff, "hey, this is working for me," etc.
Initially, the result of this meeting was called "The Manifesto for Agile Software Development." Although, with time, the name of this manifesto has been eroded and nowadays people just call it "The Agile Manifesto," this is not fair to its purpose.
Let’s stress that agile is an adjective, not a noun. So, bear in mind that there are no methods or written theory on how to develop software compliant to some "Agile" thingy. You should find your own way to discover agile practices or agile techniques to develop your software. That's the goal of the manifesto. It is not a guide or a procedure. It only contains 4 values and 12 principles that still remain valid today.
Unlike the traditional waterfall way to do software development, agile software development requires constant feedback to adapt the practices as quick as possible. Feedback is all you need to be agile.
How do you know what to do? The simple answer is, you don't. Whenever you don't know what to do—and this has nothing to do with software; this has to do with life in general—all you have to do is find out where you are, take a small step towards your goal and adjust your understanding based on what you learned. Then repeat. That is being agile.
And when faced with two or more alternatives that deliver roughly the same value, take the path that makes future change easier.
You need to know if it works as soon as possible. Make mistakes quickly so you can learn fast.
To be agile, you need to follow an empirical process, which requires trust and courage. This will bring transparency so you can truly inspect, then adapt.
As Arthur Ashe said: "Success is a journey, not a destination." And being agile is the journey we are seeking.
Inspirational source: Agile is Dead (Long Live Agility) by Dave Thomas.