To lessen means to make something smaller, weaker, or less intense. When you turn down the volume on a speaker, you lessen the sound. When a storm begins to weaken, the wind and rain lessen. When you take medicine for a headache, it lessens the pain.

The word applies to anything that can decrease or diminish. A teacher might lessen your homework load if the class has a big test coming up. An apology can lessen someone's anger after an argument. Time often lessens grief: the sharp pain of losing a beloved pet doesn't disappear, but it gradually lessens as happy memories begin to replace the initial sadness.

Notice that lessen is different from lesson (something you learn), even though they sound identical. When you remember that less is the opposite of more, you'll know that lessen means to make something less than it was before. If your enthusiasm for a project starts to lessen, you're becoming less excited about it than you were at the beginning.