We use the term Arctic all the time, and you've probably heard of the Arctic Circle.  But what is the Arctic Circle exactly?  

It actually has a very precise astronomical definition.  It is the latitude that marks the limit of where the sun remains continuously above the horizon on the summer solstice and never rises above the horizon on the winter solstice.  Here is the math: 90°- 23°26' (the tilt of the earth) = 66°34' (latitude of Arctic Circle).  

As you look at the picture below, imagine how as the Earth turns any point above the Arctic Circle would have continuous daylight or darkness on the solstices.