Agile is a mindset informed by a set of values and principles. It is not a methodology. Within the Agile framework are methodologies which prescribe practices to manage expectations and delivery and respond to changing scope or priorities. But which methodology is right for a given situation?
Adjustable scope/priorities, no schedule → Kanban.
Adjustable scope/priorities, fixed or adjustable schedule, dedicated resources of fewer than ~50 people → Scrum.
Adjustable scope/priorities, fixed or adjustable schedule, dedicated resources of greater than ~50 people who must plan, commit, and execute on a single backlog → Scaled Agile.
Fixed scope, adjustable schedule or resources → Waterfall.
Fixed scope, fixed schedule, fixed resources → House of Pain.