Appendix A. Glossary
This appendix contains informal definitions for many of the terms you will encounter. Some of these terms have to do directly with the Objective-C language itself, whereas others gain their etymology from the discipline of object-oriented programming. In the latter case, the meaning of the term as it specifically applies to the Objective-C language is provided.
A class defined to make creating subclasses easier. Instances are created from the subclass, not of the abstract class. See also concrete subclass.
A method that gets or sets the value of an instance variable. Using accessor methods to set and retrieve the values of instance variables is consistent with the methodology of data encapsulation. ...