On MH, we generally stick to using "squib" to describe the offspring of two pureblood parents. However, other sources (off MH) that I've found say that squibs are the offspring of one or more magical parents. Honestly, I'd say go with whatever makes you more comfortable. And yes, squibs are very rare, and the sources I've found state that they occur in 1:100 at the most, 1:1000 at the least.