Google Definition: 1) each of a set of values of parameter of which a differential equation has a nonzero solution (an eigenfuction) under given conditions. 2) any number such that a given matrix minus that number times the identity matrix has a zero determinant.
reference: https://www.khanacademy.org/math/linear-algebra/alternate-bases/eigen-everything/v/linear-algebra-introduction-to-eigenvalues-and-eigenvectors
Kaiser-Guttman Criterion: ‘Eigenvalues greater than one’ (Guttan, 1954; Kaiser, 1960, 1970) is commonly used to determine number of factors to retain. The thinking behind the criterion is “that a factor must account for at least as much variance as an individual variable” per Nunnally and Bernstein (1994)