Cushioning (or shock absorption): the ability of a shoe to absorb the impact of a foot-strike.
Lateral: referring to the outer side of a shoe.
Medial: referring to the inner side (or arch side) of a shoe.
Midsole: the area of the shoe between the upper and the outsole that's primarily responsible for the shoe's cushioning. Most midsoles are made of foams: either EVA (ethylene vinyl acetate) or polyurethane. EVA is lighter and more flexible than polyurethane, but it also breaks down more quickly. Many midsoles also have additional cushioning elements such as air, gel and various embedded plastic units.
Motion control: the ability of a shoe to limit overpronation.
Outsole: the material, usually made of hard carbon rubber, on the bottom of most running shoes; the layer of the shoe that contacts the ground.
Overpronation: the excessive inward roll of the foot before toe-off. It is believed to be the cause of many running injuries.
Post (or medial post): firmer density of midsole material added to the inner side of the shoe. A post is designed to reduce overpronation.
Ride: the ability of a shoe to provide a smooth transfer of a runner's weight from heel-strike to toe-off. Ride is a largely subjective quality, but shoe wearers know when a shoe has or lacks a good ride.
Stability: the ability of a shoe to resist excessive foot motion.
Toebox: the front portion of a shoe's upper. A wide toebox allows plenty of room for the toes to spread.
Upper: the leather or mesh material that encloses the foot.