Lets get technical

In Objective-C, NS stands for NeXTSTEP , the platform for which these classes of the Foundation Framework were originally designed. While it helps, but I like it when it is Number instead of NSNumber , and Array instead of NSArray .  Similarly, IB in IBOutlet and IBAction stands for Interface Builder which is the tool used for designing the user interface of your apps. The interface builder file has the extension .xib (originally .nib) NeXTStep was an object oriented operating system based on UNIX. Remember the story of Steve Jobs getting sacked from Apple, and then founding NeXT and then folks in Apple begging him to come back? Just makes you wonder how amazing Steve was. Something developed in the 1980s to 1990s becomes the building block for most of the apps built on iOS and OSX today. Simply brilliant!

In computer  programming , a  sentinel  value (also referred to as a flag value, trip value, rogue value, signal value, or dummy data) is a special value in the context of an algorithm which uses its presence as a condition of termination, typically in a loop or recursive algorithm. More info on the same here  https://en.wikipedia.org/wiki/Sentinel_value In Objective-C it looks something like this where 'nil' is the sentinel. NSArray *arrayElements = [ NSArray arrayWithObjects : @"One" , @"Two" , @"Three" , @"Four" , nil ];

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