Full Idea
An 'atom' in mereology is an individual with no proper parts. We shall use the expression 'At x' to mean 'x is an atom'.
Gist of Idea
Atom: an individual with no proper parts, written 'At x'
Source
Peter Simons (Parts [1987], 1.1.11)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.16
A Reaction
Note that 'part' in standard mereology includes improper parts, so every object has at least one part, namely itself.