It's an array. A 3d array to be precise, but just an array.
A vector (as defined by isvector(...) )is an nx1 or 1xn array
A matrix (as defined by ismatrix(...) ) is an nxm 2d array (so a vector is a subset of a matrix as a 1d array is not possible in Matlab).
You can also specify a row or column vector explicitly with isrow(...) and iscolumn(...)