Thereby means “by that action” or “as a result of that.” It connects one action to its direct consequence. When a scientist discovers a new medicine and thereby saves countless lives, the discovery is what makes the saving possible. When you study hard for a test and thereby earn a good grade, your studying directly leads to that result.

The word appears often in formal writing and careful explanations. A coach might say, “Practice your free throws every day, thereby improving your game.” A teacher might explain, “The committee voted to extend recess, thereby giving students more time to play.” In each case, thereby signals that the second thing happens because of the first.

Think of thereby as a bridge word that shows cause and effect in a single, elegant step. It's more sophisticated than saying “and then” or “so.” When you write, “She apologized sincerely, thereby mending their friendship,” you're showing that the apology itself led to the mending. The word helps you write precisely about how one action leads to a specific outcome, making your explanations clearer and more powerful.