I would like to challenge you all to define quantum mechanics. An ideal definition should contain only one sentence and should cover all aspect of quantum mechanics. If you cannot construct an ideal definition, you can try with a good but not ideal one which covers most but not all aspects of quantum mechanics, and/or contains more than one sentence. Here is my try: Definition 1 (probability amplitude): Probability amplitude is a complex number the squared absolute value of which is equal to probability. Definition 2 (quantum mechanics): Quantum mechanics is a branch of physics which studies probabilities of different measurement outcomes, for systems for which the probability can be most easily calculated from a probability amplitude.