Merriam-Webster Collegiate Dictionary - quantum

 I. noun  (plural quanta)  Etymology: Latin, neuter of quantus how much  Date: 1567  1.  a. quantity, amount  b. portion, part  c. gross quantity ; bulk  2.  a. any of the very small increments or parcels into which many forms of energy are subdivided  b. any of the small subdivisions of a quantized physical magnitude (as magnetic moment)  II. adjective  Date: 1942  1. large, significant a ~ improvement  2. of, relating to, or employing the principles of ~ mechanics ~ physics