Agile is many things, and different people see it through different lenses.
Dave Thomas, one of the authors of the Agile Manifesto, said in a talk that “agile” is an adjective – not a noun – and people should use it as such. But alas, popular use dictates a word’s status, and “Agile” is widely used as a noun. So while “agile” is still an adjective, “Agile” is a noun, whether Dave wants it to be or not.
But what is it? The Agile movement arguably began with the publication of Kent Beck’s seminal book Extreme Programming Explained in 1999. That book sent shock waves through the IT industry, and stimulated a-lot of debate about the methods being used to build software, and how, in fact, software should be built. Two years later a group of IT professionals decided to meet and hammer out some ideas, and their product was the “Manifesto for Agile Software Development”, which has come to be called the “Agile Manifesto”. That is when the term “Agile”, in capitals, started to take root (it had been used by some people before that), and the term generally encompasses “Extreme Programming”, as well as the Agile Manifesto, and many other things that have arisen since then.
Today, the phrase “Agile software development” is also a widely used term (it has a Wikipedia page), and the term “Agile” is often shorthand for that.
So “Agile” can refer to the ideas expressed in the Agile Manifesto, or it can refer to Agile software development. It can also refer to the many methods – commonly called “practices” – that are advocated by those who follow Agile, which is often referred to as the “Agile community”.
So the term “Agile” can be any or all of these.
And the word “agile” is still just an adjective.