The term 'diamond flaw' is a phrase used to describe a defect or imperfection in a diamond. It can refer to any type of flaw, such as an inclusion, a chip, or a crack. The crossword solution for this term is 'ERROR', which is a general term for a mistake or a fault. This term is often used to describe a flaw in a diamond, as it implies that the flaw was caused by human ERROR rather than a natural occurrence.