Full Idea
An arbitrary sequence of objects which satisfies every sentential function of the sentences L' will be called a 'model' or realization of the class L of sentences. There can also be a model of a single sentence is this way.
Gist of Idea
A 'model' is a sequence of objects which satisfies a complete set of sentential functions
Source
Alfred Tarski (The Concept of Logical Consequence [1936], p.417)
Book Reference
Tarski,Alfred: 'Logic, Semantics, Meta-mathematics' [Hackett 1956], p.417
A Reaction
[L' is L with the constants replaced by variables] Tarski is the originator of model theory, which is central to modern logic. The word 'realization' is a helpful indicator of what he has in mind. A model begins to look like a possible world.