A puzzle solved through an indirect and creative approach, using reasoning that is not immediately obvious and involving ideas that may not be obtainable only using logic.
A lateral thinking puzzle is a puzzle that involves thinking in ways outside of a challenger's preconception – that is, "thinking outside the box". The solution is usually obtained 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.
When writing a lateral thinking puzzle (this goes for all puzzles on the site, but especially for this genre), one must take care to compose it in such a way that there is a unique correct answer, otherwise the question is likely to be closed as "too broad".
The term lateral thinking was coined in 1967 by Edward de Bono in his book "Lateral Thinking: Creativity Step by Step" (Harper & Row publishers). The following riddle (due to Edward de Bono) is typical for this puzzle type:
A boy and his father are driving on a road when suddenly their car crashes. They are both severely injured and taken to a hospital. When they arrive in the operating room, the surgeon says, "I can't operate on this boy, he's my son!" How is this possible?
The surgeon is the boy's mother. The difficulty of the riddle depends on the reader assuming that a surgeon is male, and therefore wondering how the boy's father could be in any condition to operate on anybody when he arrived severely injured in the same operating room.