Explicit Dependency: an explicit dependency exists between two objects if one of them requires a symbol directly provided by the other.
Implicit Dependency: an implicit dependency exists between two objects if one of them requires a symbol provided by a third object, which either explicitly or implicitly requires the other object.
Undefined Symbol: an object that uses a function or data provided by another object will contain an undefined symbol for that function or symbol, which is resolved at link time with the providing object.