An atom is the smallest particle of an element. Atoms are the building blocks of the universe. Atoms can exist alone, but they also combine with other atoms to form chemical elements. Atomic is often used to describe something tiny and indivisible. Clear uses atomic as an analogy to describe something imperceptible that forms with other tiny, imperceptible changes to enact larger transformations over time, writing “an atomic habit is a little habit that is part of a larger system. Just as atoms are the building blocks of molecules, atomic habits are the building blocks of remarkable results” (Chapter 1, 27).

A compound describes two or more atoms that are fused together. Clear uses compound as an analogy to describe how the amalgamation of atoms—or atomic improvements in habits—forms something bigger over time. For example, compound interest describes returns on both the principle of investment and the interest. It is interest on interest. To apply this to habit formation, Clear suggests that improvement also happens in compounding ways; you improve on top of your previous improvements, not your original baseline.