Henceforth means from this moment forward. When a king declares that henceforth all citizens will have three days off for the harvest festival, he means starting now and continuing into the future. When your teacher announces that henceforth all homework must be turned in by 3 PM, she means from today onward, with this rule applying to all future assignments.

The word has a formal, official ring to it. You probably wouldn't say, “Henceforth I'm having pizza for lunch,” but you might write in your journal, “I tried staying up late to read, but I was exhausted at school. Henceforth, I'll finish my book on weekends instead.” The word suggests you're making a real decision or announcing an actual change, not just a passing thought.

Henceforth appears often in legal documents, historical proclamations, and formal announcements because it marks a clear turning point. The Emancipation Proclamation declared that henceforth, enslaved people in certain states would be free. When something changes henceforth, there's a before and an after, and the word draws a line between them.