The term 'Toughen by Heat' can be summarized by the crossword solution 'ANNEAL, ' which is a process of heating and cooling metal or glass to make it stronger and more durable. Annealing is often used in metallurgy and glassmaking to reduce the hardness of a material and improve its ductility, making it easier to work with. It also helps to reduce internal stresses and prevent cracking or warping.