For example, a data set consisting of the number of wins for a single football team at each of several years is a single-dimensional (in this case, longitudinal) data set.
A data set consisting of the number of wins for several football teams in a single year is also a single-dimensional (in this case, cross-sectional) data set.
A data set consisting of the number of wins for several football teams over several years is a two-dimensional data set.
[1] While, strictly speaking, two- and higher-dimensional data sets are "multi-dimensional", the term "multidimensional" tends to be applied only to data sets with three or more dimensions.
The three dimensions provide more information than can be gleaned from two-dimensional panel data sets.