Molecular blades are blades which come to a molecule-thick edge, as the name suggests. While this can make very precise cuts, there is a problem that the blades are fragile and can dull quickly or break easily. Various solutions to the problem exist; a common method is to magically reinforce the material the blade is made from. Another, detailed here, is to make the molecule-thin blade retractable.
On the surface, a knife with a retractable molecular blade ("remolec" for short) appears to be a normal knife; indeed, it can be used normally without extreme risk of cutting. This is because the remolec is stored inside the "normal" blade. At the press of a button (or on verbal command, depending on the make of the product), the molecular blade is extended, giving the "normal" blade a razor-sharp edge. With another press or command, the blade is retracted.
They are popular with surgeons and scientists. Many assassins, bounty hunters and soldiers also carry weapons with built-in remolecs.