Terminology glossary

Short definitions intended for interview recall. Each term should eventually link to its deeper chapter.

Term	Precise shorthand
Abstraction	A boundary exposing relevant behavior while hiding implementation decisions.
Aggregate	A DDD consistency boundary with one entry point, the aggregate root.
Cohesion	How strongly the responsibilities inside a module belong together.
Coupling	The degree to which one component depends on another’s existence or details.
Dependency injection	Supplying collaborators from outside an object instead of constructing or locating them internally.
Entity	An object defined primarily by identity and continuity, not only its attributes.
Idempotency	Repeating an operation has the same intended effect as performing it once.
Invariant	A condition that must remain true for a model or operation to be valid.
Isolation level	The database guarantees governing which concurrent transaction effects may be observed.
Late static binding	PHP’s static:: resolution against the runtime-called class rather than the declaring class.
Polymorphism	Different implementations satisfying the same usable contract.
Referential transparency	An expression can be replaced by its value without changing program behavior.
SRP	A module should group behavior that changes for the same reason; it is not “one method per class.”
Value object	An immutable concept defined by its values, equality rules and invariants rather than identity.
Variance	Rules governing how subtype relationships apply to parameter and return types.