Cohesion
In plain terms
How focused and related the responsibilities within a module are.
Going deeper
High cohesion means a module does one well-defined thing, making it easier to understand and maintain. Low cohesion mixes unrelated tasks. Good design pairs high cohesion with low coupling.
Under the hood
High cohesion (functional) is the goal; low cohesion (coincidental, logical) signals a class doing too much. Pairs with low coupling for maintainability.
Examples
A class that only handles invoices, single-purpose modules, focused functions.
Think of it like...
Like a well-organized toolbox where each drawer holds one kind of tool, rather than a junk drawer of unrelated odds and ends.