An n*n matrix is a matrix with n rows and n columns.
An n*n matrix is a linear transformation of n-dimensional Euclidean space. An EigenVector of an n*n matrix is a vector that gives a direction in which that transformation is simply a scaling. The amount of scaling is the associated EigenValue. This is incredibly valuable because it lets you do MatrixFactoring and MatrixAnalysis.
See also http://foldoc.org/?eigenvector