Full Idea
In model theory one normally defines some notion of truth in a model, and then uses it to define validity as truth in all models, and semantic consequence as the preservation of truth in models.
Gist of Idea
In model theory, first define truth, then validity as truth in all models, and consequence as truth-preservation
Source
Theodore Sider (Logic for Philosophy [2010], 10.1)
Book Reference
Sider,Theodore: 'Logic for Philosophy' [OUP 2010], p.254