A cluster is a group of similar things gathered closely together. When you look up at the night sky, stars often appear in clusters rather than spread evenly apart. Grapes grow in clusters on the vine, each bunch holding dozens of individual grapes packed tightly together. In your classroom, you might notice a cluster of desks pushed together for group work, or a cluster of students gathered around an interesting science experiment.

The word emphasizes both closeness and similarity. Random objects scattered across a room aren't a cluster, but a collection of seashells grouped on your dresser is. Scientists study galaxy clusters, where hundreds of galaxies orbit near each other in space. Doctors talk about disease clusters when several cases appear in the same area. Towns sometimes grow in clusters along a coastline or around natural resources like rivers.

You can also use cluster as a verb: “The kindergarteners clustered around the teacher during story time.” The word captures that natural tendency of similar things to gather together, whether it's bees clustering around their hive, houses clustering to form a neighborhood, or ideas clustering together in your mind as you work through a problem.