Mereological nihilism (also called compositional nihilism, or rarely simply nihilism) is the position that objects with proper parts do not exist (not only objects in space, but also objects existing in time do not have any temporal parts), and only basic building blocks without parts exist. Or, more succinctly, "nothing is a proper part of anything."

Almost everyone knows what a part and a whole are; they are some of the first concepts that children or infants learn. A ball is made up of two halves, so the ball is a whole that is made up of two parts. Every single object we experience in the world outside of us and around us is a whole that has parts, and we never experience an object that does not have parts. For example, a tail is a part of a lion, a cloud is a part of a greater weather system or, in visual terms, the sky, and a nucleobase is a part of a DNA strand. The only things we know of that do not have parts are the smallest items known to exist, such as leptons and quarks, which can't be 'seen', so are not experienced—at least not directly, but indirectly through emergent properties. Thus all objects we experience have parts.
A number of philosophers have argued that objects that have parts do not exist. The basis of their argument consists in claiming that our senses give us only foggy information about reality and thus they cannot be trusted; and for example, we fail to see the smallest building blocks that make up anything, and these smallest building blocks are individual and separate items that do not ever unify or come together into being non-individual. Thus they never compose anything. So, according to the concept of mereological nihilism, if the building blocks of reality never compose any whole items, then all of reality does not involve any whole items, even though we may think it does.