Momentous means having great importance or significance, especially because something will change or be remembered as a result. When an event is momentous, it marks a turning point or milestone that matters deeply.

The day you learn to ride a bike might feel momentous to you because you've accomplished something you've worked toward for months. A momentous decision is one with serious consequences: choosing which high school to attend, or a family deciding to move to a new city. Historical events like the signing of the Declaration of Independence or the first moon landing were momentous because they changed the course of history.

The word carries a sense of weight and permanence. Not every important event is momentous. Your weekly spelling test matters, but it's not momentous. A momentous occasion stands out as particularly significant, the kind of experience you'll remember and talk about years later. When something momentous happens, you often feel it in the moment: a sense that what's occurring right now will echo forward through time.