What is the standard for identifying a deformity or defect in teeth, and is "al-tafalluj" (gaps between teeth) considered a defect or a deformity?

It is permissible to straighten teeth if the gap between them is unsightly and causes a clear defect, in order to remove this blemish. However, if the purpose of straightening is solely to increase beauty and attractiveness, then it is not permissible. The criterion for this is what people consider a disfiguring defect that they find ugly, such as a large gap between teeth, as opposed to a slight gap which is considered a sign of beauty.