Logical Data Modeling - (Atomic|Indivisible)
About
Atomic means indivisible.
Atomic data is the most detailed information collected.
Such data cannot be subdivided further and is said to be an the lowest grain.
In data analysis, this is in general data that represents an event or a dimension (ie entity).
If you drill down in your data (ie by adding a column to your query), the returned row set should have the same count than before.
The atomic grain of a set is defined by its unique key (a primary key is also a unique key)