To make a plan, a decision, or an idea official, especially by deciding and clearly describing all the details. After months of talking, a group may finally formalize the agreement. You can also formalize a relationship — make it officially a job, a partnership, a marriage. Researchers formalize a theory when they pin it down so other people can test it. It is fairly formal English; in everyday talk people often just say “make it official” or “write it down.” Though actually, it does not mean “make more polite” or “dress it up.” The heart of the word is: the details are now decided, and everyone is supposed to treat them as settled.