Agile is a set of values and principles that provide the basis for multiple development frameworks used to manage the development of technology products primarily, e.g., Scrum. Agile was first codified in the Agile Manifesto, an artifact everyone involved in technology product development should read. Agile can be thought of as a response to low success rates in software development and inflexible processes associated with the Waterfall approach to managing software projects (like product releases). The authors of the Agile Manifesto published a list of related principles.
Agile is often confused with the development frameworks it has inspired, especially Scrum. Agile IS NOT a development methodology or framework; it is a set of values (from the manifesto) and the associated principles.