Given an element $a$ of a Boolean algebra (or other poset) $A$, recall that $a$ is an atom in $A$ if $a$ is minimal among non-trivial (non-bottom) elements of $A$. That is, given any $b \in A$ such that $b \leq a$, either $b = 0$ or $b = a$.

A Boolean algebra $A$ is atomic if we have $b = \bigvee_I a_i$ for every $b \in A$, where $\{a_i\}_I$ is some set of atoms in $A$.