Final answer:
The access modifier that can be accessed only by code in the same class or derived classes is the 'protected' access modifier, used in object-oriented programming to restrict access to certain class members.
Step-by-step explanation:
The access modifier that can be accessed only by code in the same class or struct, or in a class that is derived from that class, is known as the protected access modifier. In object-oriented programming, specifically within languages like C# and Java, the protected modifier allows a class to restrict access to its members (such as methods and variables) such that only its own instances and those of its subclasses can access them. This is particularly useful when you want to control the level of access that inherited classes have to the parent class's members, without making them available to the rest of the program.
The protected access modifier restricts access to the member only within the class and its derived classes, allowing code from other classes or structs to access it only if they inherit from the same class.
For example, consider a base class called Animal. If a derived class called Dog is created, the protected members of the Animal class can be accessed within the Dog class, but not by code in any other class that doesn't derive from Animal.