“Ordinary, simple humans. Muggles. They go about their lives with some part of them knowing there is something more, just below the surface: like blood. They choose to ignore it and pretend that it isn’t there. There is no magic, no miracles, and no bogeymen to grab them while they sleep.
Let us enlighten them.”
Ordinary humans, muggles as wizards call them, number over six billion stretched all over the globe. Most of them have no discernible special abilities and don’t know about the world beyond their cities and towns.
In Beyond the Veil, most muggles don't know anything about magic beyond top hats and white rabbits. Recently Lord Voldemort has been stirring up trouble in the U.K., so it is a reasonable conclusion that at least some muggles are suspicious of something odd. Also, some muggles know exactly what's going on, such as those who have children that are magical.
It is conceivable that some muggles know about wizardkind without having children at the school or being married to a witch or wizard. There may be sloppy witches or wizards or perhaps a confidant. However, please do not make a muggle expecting this without a very compelling and believable biography. Muggles that would know without family members being involved would be extremely rare.