What is an actual name of instance variable, say, topSpeed, as from lectures of Stanford University about the Objective-C and iOS development?

Here is the code: 

@property (nonatomic) double topSpeed;

Looking at this code I will think that I have defined a variable topSpeed in the class. I can't understand why it will declare automatically the getter method with the name the same as the variable name - topSpeed?

Another question is when we use 

@synthesize topSpeed = _topSpeed

And if we look at what the @synthesize will generate: 

- (double) setTopSpeed:(double)speed
{
   _topSpeed = speed;
}


- (double) topSpeed
{
   return _topSpeed;
}

What is _topSpeed here and what is topSpeed? I have declared a variable topSpeed, not the _topSpeed. What if I don't use property what would the variable name be?

objective-c properties
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