A pact is a formal agreement between two sides — countries, groups, or sometimes people — usually to help each other or to stop fighting. It is heavier than a casual “I promise.” When newspapers say two countries signed a pact, they mean a serious deal, often written down: a peace pact, a defence pact, a pact between political parties. The word feels a little formal, and a little solemn next to deal or agreement, which is why writers like it when they want a serious tone. Though actually, people stretch it. Friends may joke about a pact to go to the gym every morning, and that is play, not a legal paper. The tidy picture in the dictionary is the public, formal kind. In modern English it is almost always a noun. Using it as a verb is rare, and you can ignore that for now. A treaty is often the bigger, more official international kind; pact is a wider word and can sound slightly looser, though still serious.