A state of not knowing what to decide; a state of difficulty or perplexity; a state of uncertainty, hesitation or puzzlement. / A dilemma, a difficult decision or choice.

(quantum mechanics) One of certain integers or half-integers that specify the state of a quantum mechanical system (such as an electron in an atom).

(quantum mechanics) The discontinuous change of the state of an electron in an atom or molecule from one energy level to another. / (figuratively) An abrupt, extreme change.

(physics) The branch of physics which studies matter and energy at the level of atoms and other elementary particles, and substitutes probabilistic mechanisms in place of classical Newtonian ones.

Argumentative; fond of or prone to quarreling.

Involving four elements.

A serving of four shots of espresso. / (chess) A kind of round-robin tournament between four players, where each participant plays every other participant once.

Having four shots of espresso.

(informal) A quadrangle (quadrangular courtyard). / (informal) A quadruplet (infant). / (informal, computer graphics) A quadrilateral. / (informal) The quadriceps muscle.

(poker slang) Of or relating to quads.