What's the difference between reaction norms and phenotypic plasticity?
Phenotypic plasticity, as you define in your question, is an ability or a property of the organism (a given genotype) to manifest different phenotypes in different environments (Pigliucci et al., 2006).

Reaction norm is an analysis of the pattern of phenotypes exhibited in different environments. In other words it is an analysis of the phenotypic plasticity.