Definition:Simple Statement

Definition
A simple statement is a statement which has one subject and one predicate.

For example, the statement:
 * London is the capital of England

is a simple statement.

London is the subject and is the capital of England is the predicate.

Atoms in Propositional Logic
Simple statements are considered as atomic in propositional logic.

Thus, in that context, simple statements are frequently referred to as atoms.

Hence some sources refer to a simple statement as an atomic statement or atomic sentence.