A dilemma (Greek δί-λημμα "double proposition") is when two solutions or possibilities are present. If both solutions are undesirable or impossible, this may put a person on the horns of a dilemma, neither outcome is comfortable.

The dilemma is sometimes put as a choice "you must accept either A, or B". Here A and B are each leading to some undesirable conclusion.

In formal logic, the definition of a dilemma differs from everyday usage. Two options are still present, but choosing between them is immaterial because they both imply the same conclusion. Symbolically expressed thus:

This can be translated informally as "one (or both) of A or B is known to be true, but they both imply C, so regardless of the truth values of A and B we can conclude C."

Horned dilemmas can present more than two choices. The number of choices of horned dilemmas can be used in their alternative names, such as two-pronged (two-horned) or dilemma proper, or three-pronged (three-horned) or trilemma, and so on.

Constructive dilemmas

1. (If X, then Y) and (If W, then Z).
2. X or W.
3. Therefore, Y or Z.

Destructive dilemmas