What is the precise definition of, "Differs in exactly one element"?

[Jarvis323]

Do {1,2,3} and {1,2} differ in, or by exactly one element? Or do those two things mean the same thing. Also, what about {1,2,3} and {1,2,4}.

 

[pbuk]

If you want to be precise use symbols instead of words, but in normal usage:
{1,2,3} and {1,2} differ by exactly one element
{1,2,3} and {1,2,4} differ in exactly one element