Rejoicing is what you call it when a lot of people are very happy together because something good has happened — a war ending, a team winning, a baby coming after a long wait, good news for a whole town. It is less “I feel happy” and more “the place is full of glad noise.” You will meet it in stories, in news about celebrations, and in slightly old-fashioned writing: “there was great rejoicing in the streets.” In everyday talk, people more often say everyone was celebrating, or everyone was so happy. Though actually, a person can rejoice quietly too (that is the verb), but this noun usually pictures a shared, public kind of joy, not one person smiling at a phone.