Axis: An axis is a means of providing information about the characteristics of a fact reported within a financial report.

An [Axis] is one of multiple means of modeling the characteristics of a fact reported within a financial report. Other means of modeling characterisics are the entity/identifier, period, and line items.

Note that the XBRL technical syntax uses the term dimension and the US GAAP Taxonomy uses the term [Axis] to describe the same thing.