Sept. 21 is the U.N. International Day of Peace, but how does the world measure peace, exactly? In the dictionary, you’ll find the word “peace” nestled between “pea” and “peach.” The formal definition will refer to peace as the absence of violence. In other words, it’s a negative definition — explaining what peace is not, rather than what it is.

Our research on peace takes on a renewed urgency given the war in Ukraine and the uptick in the number of violent conflicts over the past decade. A number of countries — Lebanon, Colombia, Sudan and Myanmar, to name a few — appear extremely fragile and at increased risk of major violence. In times of war, or facing the threat of war, it seems sensible to think seriously about peace and how we might identify and measure it.