To quicken means to make something happen faster or to speed up. When a recipe tells you to quicken the rising of bread dough by placing it in a warm spot, you're making the yeast work faster. When your heart rate quickens during a sprint, it beats faster to pump more blood to your muscles.

You can still hear an older sense of the word in phrases like “the quick and the dead” from old stories and poems, where quick means “alive.” When a plant quickens in spring, it comes alive with new growth. When someone's interest quickens, they become more alert and engaged, like when a boring lecture suddenly includes an exciting demonstration.

Today, we mostly use quicken to mean speeding something up: your steps might quicken when you're running late, or your breathing might quicken when you're nervous. The word suggests going faster while becoming more energetic and alive in the process.