The term quantum gets abused and misused a lot these days by marketers. In its original application, the word quantum is an adjective in the phrase "Quantum Mechanics."
To understand the meaning of quantum requires understanding what scientists mean by Mechanics. Mechanics is the branch of physics that studies the motion of objects in response to forces. Basically, Mechanics is the science of how things work. It tells us what technology is possible and what technology is impossible.
What is now known as Classical Mechanics was discovered by Isaac Newton in 1687 and has, to a large extent, provided the blueprint for virtually all of our current technology since the Industrial Revolution. The exciting thing about Quantum Mechanics, which was discovered by Erwin Schrodinger in 1925, is that certain things are possible in Quantum Mechanics that are impossible in Classical Mechanics.
What this means is that there are opportunities for new technology that were overlooked before. Computers can be made that operate on different principles to significantly increase their power for certain problems. Likewise, new forms of communication are available that are impossible to eavesdrop on.
So, these days the proper definition for the word "Quantum" is to describe a new suite of technology, let’s call it Quantum Technology, that operates more efficiently than our "Classical Technology". The reason for this improvement in efficiency is that we now understand the rules of the Universe a little better, through Quantum Mechanics, and so we can optimize technology significantly.