Quantum
A quantum is the smallest increment into which many physical properties are subdivided.
Most commonly, quanta are the fundamental units of something measurable. Electromagnetic energy, for example, is quantized into photons, wavelike packets of fixed frequency. Quantum physics was founded at the beginning of the twentieth century, incorporating at a foundational level the idea that electromagnetic radiation comes in such packets; the concepts of quantum theory have proved paradoxical, and difficult to articulate in any familiar terms, but the theory built up into quantum field theory was the largest single step in the physical sciences between 1900 and 1950.
Etymology
The word quantum comes from the Latin word for "quantity".