MDArray is a multi dimensional array implemented for JRuby with similar functionalities (but still less) to numpy and narray by Masahiro Tanaka. MDArray targets specifically JRuby as it uses Java-NetCDF library from Unidata (http://www.unidata.ucar.edu).
Copying from numpy documentation but also valid for MDArray:
MDArray main object is the homogeneous multidimensional array. It is a table of elements (usually numbers), all of the same type, indexed by a tuple of positive integers. In MDArray dimensions are called axes. The number of axes is rank.
For example, the coordinates of a point in 3D space [1, 2, 1] is an array of rank 1, because it has one axis. That axis has a length of 3. In example pictured below, the array has rank 2 (it is 2-dimensional). The first dimension (axis) has a length of 2, the second dimension has a length of 3.
[[ 1., 0., 0.], [ 0., 1., 2.]]