block

As a local variable:

returnType (^blockName)(parameterTypes) = ^returnType(parameters) {...};

As a property:

@property (nonatomic, copy, nullability) returnType (^blockName)(parameterTypes);

As a method parameter:

- (void)someMethodThatTakesABlock:(returnType (^nullability)(parameterTypes))blockName;

As an argument to a method call:

[someObject someMethodThatTakesABlock:^returnType (parameters) {...}];

As a typedef:

typedef returnType (^TypeName)(parameterTypes);
TypeName blockName = ^returnType(parameters) {...};

This site is not intended to be an exhaustive list of all possible uses of blocks.
If you find yourself needing syntax not listed here, it is likely that a typedef would make your code more readable.