A puzzle that requires formal logical deduction to arrive at the solution. This suggests more than merely reasoning through clues to find an answer (you might want [situation] for that).
A logic puzzle is one where a conclusion can be deduced solely using formal logical deduction without needing to guess. It often takes the form of a series of statements that you must use to find the solution to the question posed using logical inference. For example, the puzzle might ask you to figure out the order of five people standing in line from statements like "Jamie is not to Sarah's left."
Sometimes, the truth of the statements themselves is unknown and must be deduced. Use liars in addition for these.
You are in a room with two wizards. One of these wizards always tells the truth, while the other always lies. How do you figure out which does what by asking both of them just one question?
The answer is to ask them any question that has one irrefutably true answer and one irrefutably false answer, such as "Am I speaking to you right now?" The wizard that tells the truth will say "Yes," proving that he speaks the truth, and the lying wizard will say "No," showing that he is the liar.
Note that puzzles requiring you to reason through hints or clues, rather than apply formal logic rules to supplied facts, are not logical-deduction. You might want the situation tag for those.