What is a non-magical person born to a magical family called??

Squibs. They are born into very inbred pureblood families. Due to inbreeding, the magical gene is missing or inoperative. Argus Filch and Mrs Arabella Figg are Squibs. Some suspected Merope Gaunt (Lord Voldemort's mother) to be a Squib but I don't think so as she was able to make a amor potion to bewitch Tom Riddle Sr.

They're called squibs
:) An example of one would be Filtch

A squib.

squib

squib

They're called a squib. The wizarding community consider it a disgrace to have a child that's a squib, and they usually try to cover it up, or put them on a course like Kwikspell, which helps them improve their magical abilities.