A millionth is one part of something that's been divided into a million equal pieces. If you cut a pizza into a million slices (which would be impossible but imagine it anyway), each tiny speck would be one millionth of the whole pizza. We write it as 1/1,000,000 or 0.000001.

The word helps us grasp incredibly small measurements. Scientists measure things in millionths all the time: a millionth of a second (called a microsecond) is about how long it takes light to travel about 300 meters. A millionth of a meter (called a micrometer or micron) is about the size of many bacteria. Computer processors execute instructions in millionths of a second.

You'll also hear millionth used for rankings or sequences, like “the millionth visitor to a museum” or “the millionth customer at a store,” which often wins a special prize. When something is described as a one-in-a-million chance, people mean it's extremely rare, though they're usually exaggerating rather than calculating actual odds.

Understanding millionths helps you appreciate both how vast a million really is and how scientists measure the incredibly tiny building blocks of our world.