An Accessor is (usually) a function that is responsible for reading or writing a property.
In object-oriented programming, an instance variable is a variable defined in a class (i.e
"Technically accessing the ivar directly is faster than using accessor but there are very few situations in which it will make a significant performance difference and would probably be a case of premature optimization"
from question "Self.variableName vs. _variableName vs. @sysnthesize variableName"
"Nonatomic properties don t use locks but direct ivar is faster because it skips the accessor call"
from question "Why would you use an ivar?"