Full Idea
A set of formulas of a first-order language is 'satisfiable' if there is a structure and a variable interpretation in that structure such that all the formulas of the set are true.
Gist of Idea
Some formulas are 'satisfiable' if there is a structure and interpretation that makes them true
Source
José L. Zalabardo (Introduction to the Theory of Logic [2000], §3.5)
Book Reference
Zalabardo,José L.: 'Introduction to the Theory of Logic' [Westview 2000], p.106