Full Idea
All occurrences of variables in atomic formulas are free.
Clarification
'Free' is the opposite of 'bound'
Gist of Idea
All occurrences of variables in atomic formulas are free
Source
John P. Burgess (Philosophical Logic [2009], 1.7)
Book Reference
Burgess,John P.: 'Philosophical Logic' [Princeton 2009], p.8