Lateral thinking: a way of thinking which seeks the solution to intractable problems through unorthodox methods, or elements which would normally be ignored by logical thinking.

1966 London Life 22 Oct. 20/3 He [sc. Dr. Edward De Bono] divides thinking into two methods. One he calls ‘vertical thinking’—that is, using the processes of logic, the traditional-historical method.‥ The other type he calls ‘lateral thinking’, which involves disrupting an apparent sequence and arriving at the solution from another angle.