Longman Dictionary of Contemporary English
Wiktionary
n. Using reasoned thought in a non-standard, or non-linear logical, way to find a solution to a problem.
WordNet
n. a heuristic for solving problems; you try to look at the problem from many angles instead of tackling it head-on
Wikipedia
Lateral thinking is solving problems through an indirect and creative approach, using reasoning that is not immediately obvious and involving ideas that may not be obtainable by using only traditional step-by-step logic. The term was coined in 1967 by Edward de Bono.
According to de Bono, lateral thinking deliberately distances itself from standard perceptions of creativity as either "vertical" logic (the classic method for problem solving: working out the solution step-by-step from the given data) or "horizontal" imagination (having many ideas but being unconcerned with the detailed implementation of them).