Arch: In an arch a gymnasts hips are pushed forward, chest is open. Lie on your stomach with your arms by your ears. Lift your heels while keeping your legs straight, and lift your arms while keeping your arms straight.
Bridge: A bridge is attained by lying on your back. Place your hands on the floor by your ears and bend your legs. Push your hips towards the ceiling and arch back. Ideally a bridge should have straight legs and shoulders pushed out over the hands.
Candlestick: A candle stick is a position where the gymnast is essentially standing on the back of their shoulders with their feet pointed towards the ceiling. The gymnasts arms can either be by their head, or back pushing on the floor to assist with support and balance.
Handstand: A proper handstand is extended towards the ceiling, shoulders are open, body is hollow.
Head In/Out: A gymnasts head is "in" when their chin is tucked on their chest, or close to. A gymnasts head is out when their head is tilted back.
Hollow: In a hollow a gymnasts hips are turned under, legs are tight chest rounded inward. Lie on your back on the floor with your arms by your ears. Lift your legs slightly off the ground. Lift your head slightly off the floor. Your lower back should maintain contact with the floor.
Layout: In a layout a gymnast is not bent at the hips nor the legs. A layout is the term used for a rotating skill in which the gymnasts body is essentially straight. A layout can be performed either hollow or arched.
Lunge: To do a lunge start standing feet together. Take a large step forward. Bend your front leg. Both feet should be turned out somewhat. Arms should be extended upwards so that the line from the rear foot to the hands is straight.
Open/Closed Hips: Opening the hips is proceeding towards an arched position. Completely closed hips is a fully piked position where the gymnasts chest is flat agains their legs.
Open/Closed Shoulders: Completely closed shoulders is defined as your arms being down so that your fingers are touching your legs. To "open" your shoulders lift your arms straight out in front and continue upwards until your hands are pointed straight at the ceiling.
Overgrip/Undergrip: Overgrip is gripping a bar so that your palms are facing the same direction as your face. Undergrip is gripping a bar so that your palms are facing the opposite direction as your face.
Pike: In a pike a gymnast is bent only at the hips. Sit on the floor with your legs straight out in front of you. Pikes of varying degrees including where a gymnast is esentially folded in half at their hips are used in gymnastics.
Planch: This is a handstand in which the body is parallel with the ground.
Puck: A puck is a cross between a pike and a tuck. It's pretty much a somewhat open tuck position, or a pike with moderately bent knees.
Split, Side or Front: In a side split one leg is forward, the other leg back. Hips are kept as square as possible. To get the splits or other flexibility it is more important to stretch often than to stretch for a long time in one sitting. Stretch every day.
Straddle: In a straddle a gymnasts legs are separated with neither leg being forward or backward of the other. A straddled pike is a straddle in which the hips are closed or "piked" to some degree.
Tuck: In a tuck a gymnast is bend at the hips and the knees. Sit on the floor with your legs in front of you. Bend your knees so that your knees are touching your chest and your feet are "tucked" in close to your body. A variation on the tuck is called a "cowboy" tuck in which the gymnast pulls their knees out to the side somewhat in order to compress the tuck further. This enables faster rotation.