Quantum (plural: quanta) is a word for a minimal amount of something: in physics, specifically quantum mechanics, it is the minimum amount that can result in certain actions taking place. The word clearly borrows on the same word-root as the word "quantity", representing a kind of minimum quantity. An example quantum is the minimal amount of electromagnetic radiation's (EMR's) energy at a given frequency.
Such a minimum amount of EMR can be treated as a particle (termed a photon), and a beam of EMR acts like a stream of such photons. However, some situations famously show one of these "particles" to be passing through both of two side-by-side holes in a barrier simultaneously, a situation where considering EMR to be a wave spanning both holes offers a more straight-forward description.
The term, quantum is also often used to indicate "having to do with quantum mechanics", e.g., quantum computing, which uses quantum mechanical phenomena to help carry out calculations.