An axis of an array is called a dimension.
Each dimension is a non-negative fixnum; if any dimension of an array is zero, the array has no elements. It is permissible for a dimension to be zero, in which case the array has no elements, and any attempt to access an element is an error. However, other properties of the array, such as the dimensions themselves, may be used.
188.8.131.52.1 Implementation Limits on Individual Array Dimensions