“Single-crystal” means the blade is cast rather than machined/forged, and cooled rapidly enough that the molten metal doesn’t have time to form a crystalline structure - the whole thing is one big metal crystal.
Thanks to you and NorthMountain for the explanations. I am a regular viewer of “Forged in Fire”, but must have missed that day! I guess it’s the difference between one single crystal and an entire lattice structure, then? I can see how that would be much stronger and it makes sense for turbine blades (and knives and swords). I once read an excellent book by a test pilot where they had blade failures and they found it was from impurities when they poured the molten metal into molds. The blades that were failing were almost always the last to be cast...