Reflection is a property which causes magical rays, such as cold, fire, sleep, lightning, death and disintegration to bounce back. It also protects you from Medusa's stoning gaze and the floating eye's paralysis attack. It is normally obtained extrinsically through various items, although polymorphing into a silver dragon or baby silver dragon gives you the property intrinsically.
Reflection can be acquired by putting on an amulet of reflection, wearing a shield of reflection, silver dragon scales, or silver dragon scale mail, or wielding The Longbow of Diana, the Ranger quest artifact.
The top level of Sokoban has a 50% chance of containing an uncursed amulet of reflection. In the titan version of Medusa's Island, the statue of Perseus has a 75% chance of containing a cursed +0 shield of reflection. In the other version the chance is 25%.
Not only the player character and monsters can be reflective, but walls also bounce magical rays.
Reflection is a high priority trait. Usually the first time you may get it is by wishing for a silver dragon scale mail when you get a magic lamp from the Gnomish Mines. Silver dragon scale mail is especially good if your character can do two-weapon combat because then you will have both hands for your weapons, compared to wearing shield of reflection. Some weapons, such as Cleaver, are also used with two hands, preventing the use of a shield at the same time. Also, a shield will increase spell failure rates and make it tough to cast spells.
Reflection is especially important because of black dragons. A black dragon's disintegration breath will kill you instantly, unless you have reflection or disintegration resistance. It is very hard to gain disintegration resistance without killing a black dragon first, so reflection is the best option. (You might also consider genociding D).