Cohesion within functions/modules

Cohesion describes how well the operations in a function relate to one another. The best kind of cohesion is Functional cohesion, where all function operations contribute to the performance of only one task.

double square(double number)
{
return (number * number);
}

There are other weaker kinds of cohesion, the worst being coincidental. Wikipedia has a list: https://en.wikipedia.org/wiki/Cohesion_(computer_science)