"Agile" (the label) is all over the place. Who would have guessed in early 2001? When the Manifesto for Agile Software Development was created and the English adjective ‘agile’ obtained its specific meaning in the context of software development. What is this manifesto, commonly known as the Agile Manifesto, about actually?
"Scrum" (the method) is all over the place. Scrum terminology increasingly dominates software development vocabulary. Who would have guessed in 1995? When Scrum was codified for the first time and presented to the public? What is Scrum, essentially? And how does it relate to Agile?
In 4 value statements, backed by 12 principles, the signatories of the manifesto captured the mindset common to their approaches to software development. Agile is the DNA that drives behavior, not the behavior itself. Every method (behavior) claiming to be Agile can be expected to express these values and principles. The Agile Manifesto is a cohesive set of interrelated values and principles. Agile is a trait, not an activity.
With the increasing adoption of the methods for Agile software development, Agile became a proper noun, capitalized, pretty popular and ultimately big business. Success obfuscates and diminishes actionability, it seems. Today "Agile" is all over the place; coming in many flavors, wrappings, definitions, interpretations, abated and discounted. "Agile" sells. It is probably the most used prefix for roles, jobs, positions, functions and phases found in the software industry. And, it needs scaling.
Given the nature of Agile, correlating 'scaling' to Agile is pretty odd. Tactics change with scale. Strategies change with scale. Values and principles don’t change with scale. Claims and statements on the need, the ability, the inability, the whatever to scale Agile are besides the point. Values and principles are agnostic of scale.
Agility is the extension of Agile into a noun. As such it refers to the state that people, teams, organizations look for by adopting Agile development processes. Agility, as such an extension, is a state of high responsiveness, speed and adaptiveness; a state of constant invocation of change, evolution and improvement. Such state of agility augments the capability of people, teams, organizations to deal with the complexity and unpredictability that are natural to the work of software development itself, the organizational context within which it happens and the external circumstances faced. The adoption of Agile processes indeed is an important foundation for this state of (business or enterprise) agility.
Scrum emerged in the early '90s from the work of Jeff Sutherland and Ken Schwaber. They formalised and turned their way of working into a cohesive set of rules and roles for complex product development, that was formally presented as "Scrum" to the public for the first time in 1995. As the co-creators of Scrum are signatories of the Manifesto for Agile Software Development, the Agile values and principles underpin the Scrum framework.
Success obfuscates and diminishes actionability, it seems. Versions, interpretations and flavours of things that looked like Scrum started appearing, abated and discounted. It inspired the co-creators of Scrum to define Scrum, its rules and roles and how they tie together, in the Scrum Guide. Scrum as intended and designed. The core of Scrum stands, immutable, for reasons of cohesion. The tactics to apply the rules are left to the intelligence and creativity of the people using Scrum. Their unique discovery is framed and given direction by Scrum.
Scrum is a servant process, not a commanding process. Scrum thrives on empiricism and self-organization, characteristics that are better understood when seen through the lens of the Agile Manifesto.
As with Agile, the Scrum Values and Scrum's fundamental roles and rules as described in the Scrum Guide don't change with scale. But scaled implementations of Scrum do require different tactics in implementing the rules, yet respecting the foundational values and principles.
Agile and Scrum, actually, are not interchangeable synonyms, but two inseparable ingredients in a software development ecosystem, pure chemistry.
Agile are the principles to Scrum, Scrum is a foundation for agility.