Unprojection is an effort, initiated by Miles Reid, to develop an algebraic language for the study of birational geometry. [Ki] contains motivation and examples, and poses the problem of developing a general theory of unprojection. The main purpose of the present work is to suggest a general definition of unprojection, and to show that it indeed generalizes previous work done in the topic. In addition, in Section 6 we present an unprojection analysis of an example of Reid and K. Suzuki, and Section 7 contains more examples.