What does the property "Nonatomic" mean?

Take a look at the Apple Docs.

Basically, if you say nonatomic, and you generate the accessors using @synthesize, then if multiple threads try to change/read the property at once, badness can happen. You can get partially-written values or over-released/retained objects, which can easily lead to crashes. (This is potentially a lot faster than an atomic accessor, though.)

If you use the default (which is atomic; there used to be no keyword for this, but there is now), then the @synthesized methods use an object-level lock to ensure that multiple reads/writes to a single property are serialized. As the Apple docs point out, this doesn't mean the whole object is thread-safe, but the individual property reads/writes are.

Of course, if you implement your own accessors rather than using @synthesize, I think these declarations do nothing except express your intent as to whether the property is implemented in a threadsafe manner.

In addition to what's already been said about threadsafeness, non-atomic properties are faster than atomic accessors. It's not something you usually need to worry about, but keep it in mind. Core Data generated properties are nonatomic partially for this reason.

In a multi-threaded program, an atomic operation cannot be interrupted partially through, whereas nonatomic operations can.

Therefore, you should use mutexes (or something like that) if you have a critical operation that is nonatomic that you don't want interrupted.

If you specify "atomic", the generated access functions have some extra code to guard against simultaneous updates.

Usually atomic means that writes/reads to the property happen as a single operation. Atomic_operation

The "atomic” means that access to the property is thread-safe. while the "nonatomic" is the opposite of it. When you declare a property in Objective-C the property are atomic by default so that synthesized accessors provide robust access to property in a multithreaded environment—that is, the value returned from the getter or set via the setter is always fully retrieved or set regardless of what other threads are executing concurrently. But if you declare property as nonatomic like below

@property (nonatomic, retain)  NSString *myString;

then it means a synthesized accessor for an object property simply returns the value directly. The effect of the nonatomic attribute depends on the environment. By default, synthesized accessors are atomic. So nonatomic is considerably faster than atomic.

One is for multi threads. One isnt