An operational definition describes how to measure a variable or define a term. It outlines the procedures used in research.
This refers to how well a test measures what it's supposed to measure - like checking if your dish tastes as it should according to the recipe.
This is about consistency of measurement - if you follow the same recipe again, will you get the same result?
This involves repeating an experiment using the same methods but different subjects and experimenters, to see if similar results are obtained - like someone else trying out your recipe.