Complex sentence: If η is a natural transformation from "F" to "G", we also write η: "F" → "G" or η: "F" ⇒ "G".
Split into simpler sentences: If η is a natural transformation from "F" to "G", we also write or. Two functors "F" and "G" are called "naturally isomorphic" or simply "isomorphic" if there exists a natural isomorphism from "F" to "G".