Kodaira dimension
A fundamental birational invariant is the Kodaira dimension, which measures the growth of the plurigenera "P""d" as "d" goes to infinity. The Kodaira dimension divides all varieties of dimension "n" into "n" + 2 types, with Kodaira dimension −∞, 0, 1, ..., or "n". This is a measure of the complexity of a variety, with projective space having Kodaira dimension −∞. The most complicated varieties are those with Kodaira dimension equal to their dimension "n", called varieties of general type.