Cohesion means the way parts stick together or work together as a unified whole. In science, it describes how molecules of the same substance attract each other. Water droplets form round beads because water molecules have strong cohesion, pulling toward each other. This same force helps create surface tension, which lets water striders walk on ponds without breaking through the surface.

In writing and speaking, cohesion means how well ideas connect and flow together. A paragraph has good cohesion when each sentence links smoothly to the next, making the whole piece easy to follow. When you write a story, you create cohesion by using transition words like “meanwhile” or “as a result” to connect your ideas.

Teams and groups need cohesion too. A soccer team with strong cohesion doesn't just have skilled players: those players understand each other's movements, communicate well, and work toward the same goals. A class project shows cohesion when everyone contributes their part and the final presentation feels like one unified effort rather than random pieces stuck together.

The opposite of cohesion is fragmentation, where parts feel disconnected or scattered. Whether you're looking at molecules, paragraphs, or people working together, cohesion is what makes separate parts function as a strong, effective whole.