This is probably the best article for a beginner about impedance with respect to headphones.
I haven't managed to find an explanation of why the impedance of headphones is just a single, real number (like 250 Ohm). I thought impedance is a complex number, so it should have real (resistance) and imaginary (reactance) part. Maybe the imaginary part is just zero. All I know is that impedance varies with frequency.
Best Answer
Impedance does vary by frequency. However, for many things it doesn't vary very much by frequency within the expected operating frequency of the equipment, and a single number is much easier to handle than a frequency response graph. So people quote a single average number.
In fact, your linked article admits this:
That's quite a lot of variation! It also explains why this causes variation in the audio quality when driven from different sources.