It's not necessary to pull economics into the picture - the general definition is that a rational choice is making the choice that best furthers your goals. If you make suboptimal choices (with respect to your own goals - sacrificing "socially desirable" goals in order to achieve some desire of your own isn't irrational), that's an irrational choice, and people often do that for various reasons. We're not horribly bad at achieving our goals, but we're also nowhere near perfect. Some types of suboptimal choices are systematic, predictable deviations which we can study as biases.