The 2D-array is a special type of
array that represents a two-dimensional ordered sequence of scalar (i.e. non-array) data.
See also the
array for an example of a 2D-array of integers.

In Excel, it is represented by a range of cells with N₁ rows and N₂ columns.
The 2D-array can be thought as a
1D-array of which the elements are 1D-arrays
More precisely, a N₁ x N₂ range represents a 2D-array that is regarded to contain N₁ elements, each of which is a 1D-array with N₂ elements.
In other words, the convention of the data ordering in a 2D-array is first by rows and then by columns.

It is possible to specify a 2D-array of which the top level 1D-array elements have unequal size.
This is achieved by setting a few selected cells to contain the special text [end], which marks the end of the respective row, as explained at

A 2D-array can be also constructed by a single row or column.
If a function expects as input a 2D-array, then the orientation of the corresponding range is important.
For example, a single row of two cells is not the same with a single column of two cells, even if the cells contain the same data.
The former represents essentially only one 1D-array containing two values, while the latter represents a collection of two 1D-arrays, each containing one value.