The number of hairs on your head and how close they are to each other. Not to be confused with hair diameter which is the width of a single strand of hair. Hair density is on a scale from Thick to Thin.
A quick way to test is to part your hair and gently moving apart. Looking at your scalp of you can only see your scalp where the part is then you have thick hair. If you can see your scalp outside of part then you have thin hair. You may already know your hair density. What is your hair density?