Observable means able to be seen, noticed, or measured. When scientists study the observable universe, they mean all the stars, galaxies, and cosmic matter that telescopes can detect. When a doctor notes observable symptoms, she's describing signs of illness that she can actually see or measure, like a rash or fever, rather than how the patient feels inside.

The word comes from the verb observe, which means to watch carefully or notice details. Something observable stands out enough that people can perceive it with their senses or instruments. A teacher might comment on the observable improvement in a student's handwriting: the difference is clear and visible on the page.

In everyday life, we constantly make decisions based on observable facts. You know it's raining because you can observe the water falling. You know your friend is upset because you observe their frown and slumped shoulders. Scientists especially value observable evidence because multiple people can examine the same thing and confirm what they find. An observable change in a science experiment is one that everyone conducting the experiment can witness and record, making it more trustworthy and easier to check. When something is observable, it moves from the realm of hunches and guesses into the world of things we can be much more confident about.