Momentous describes an important event or moment in time. It is used for a time of great consequence or for a major accomplishment, and is almost always reserved for good things.

When a moment is so great you know you'll never forget it, you have just experienced something momentous. It can be personal — perhaps the day you were named prom queen; or something historic — like the day Elizabeth was named Queen of England. Momentous and momentary share the root word moment but momentary describes just one fleeting moment in time. A momentary occurrence can certainly be momentous, but it's not always the case.

Definitions of momentous

adj of very great significance

“a momentous event”
important, significant
important in effect or meaning

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