Cohesion is one of the noun forms of cohere; the others are cohesiveness and coherence, each of which has a slightly different meaning. Coherence is often used to describe a person's speech or writing. An incoherent talk or blog post is one that doesn't "hang together;" and if the police pick up someone who they describe as incoherent, it means he or she isn't making sense. But to describe a group or team that always sticks together, you would use cohesive, not coherent. And the words you'd use in Chemistry class to describe the way molecules hang together—for example, the way water forms into beads and drops—are cohesion, cohesive, and cohesiveness.