A flat lens is sometimes called a "window" and although it does not have a lens shape, most people still call it a lens.
Plastic Less expensive lights usually have a flat plastic lens over the reflector and bulb (or LED). Plastic lenses are more resilient than glass and can be useful if the light will get knocked around and take some abuse. However, plastic is easier to scratch and often not as clear as glass.
Sapphire Another choice is a sapphire lens made of synthetic sapphire (not actually glass) which is stronger than tempered glass and very resistant to scratches. Due to it scratch resistance, sapphire is often used for watch crystals, but is also available as a flashlight lens.
Anti-reflective coating For the same reason telescopes, binoculars, and camera lenses have coated lenses, better flashlights include a coating on the lens surface that allows more light through by preventing light from the emitter from reflecting off of the lens instead of going through it. A typical loss for uncoated glass is 4% as light enters the lens and another 4% as it passes through back into the air. However high-end coated lenses might only have a total loss of 1%. For LEDs this is like getting an extra bin of brightness from a light. Because an AR coating improves transmission of only a narrow wavelength of light, multiple layers of different thicknesses are used to get improved transmission of more wavelengths of light. Coated lenses are available from FlashlightLens.com, KaiDomain, and International Outdoor in the most common sizes, including 28mm for many P60 hosts.
Instead of a flat lens, an aspheric lens can be used that will focus the light into a tighter beam. By moving the lens with respect to the emitter, a tight beam or wider flood can be obtained. Lights with a movable lens are called flood-to-throw lights. When properly focused for maximum throw, an image of the emitter is projected showing bonding wires and strips of the LED. Though frequently called aspheric, many times the lens involved has the traditional curvilinear shape and therefore isn't really aspheric.