Cohesion

Cohesion is the degree to which the elements in a module belong together.
A module with high cohesion means that the module's elements have a high
degree of connectedness.

“Cohesion refers to what extent the parts of a module should be contained
within the same module. In other words, it is a measure of how related
the parts are to one another. Ideally, a cohesive module is one where
all the parts should be packaged together, because breaking them into
smaller pieces would require coupling the parts together via calls
between modules to achieve useful results.” Mark Richards and Neal Ford

Attempting to divide a cohesive module would only result in increased
coupling and decreased readability.
— Larry Constantine

“Software modules should aim for high cohesion.” Dianxiang Xu, p. 85

This is a foundational design principle. For example:

• Classes that violate the Single Responsibility Principle (SRP)
  have low cohesion.

• Interfaces or abstract classes that violate the
  Interface Segregation Principle (ISP) also exhibit low cohesion.

See Also:

• Abstract Classes vs Interfaces

• Big Ball of Mud

• Code Review Checklist

• Coupling

• Interface Segregation Principle (ISP)

• Modularity, Module

• Motivation for Good Design

• Pillars of OOP (Encapsulation, Abstraction)

• Single Responsibility Principle (SRP)

• SOLID Design Principles