Every activity has its own jargon and rocketry is no exception. Here are some terms in common use which will make what you read comprehensible.

• aft is the rear-ward end of something on a rocket. Ship terminology is often used because "top" and "bottom" are confusing as orientation changes. See also forward.
• airframe The rocket structure. This usually refers to just the cylindrical body tube, but may also refer to the entire body of the rocket.
• ammonium perchlorate (A.P.) Composite (mid- and high-power solid fuel) motors are generally made with ammonium perchlorate as the oxidizer and various other ingredients as the fuel.
• altimeter is a device which measures at least the maximum height a rocket reaches. These are often combined with circuitry to separate the rocket at apogee for recovery.
• apogee is the highest point of a rocket flight. An ideal rocket flight opens the rocket and ejects the recovery system at apogee.
• B.A.R. A Born Again Rocketeer is someone who flew model rockets (usually Estes) as a child and has returned to the hobby as an adult. Generally B.A.Rs. re-enter the hobby after discovering it on the Internet or after seeing the movie October Sky.
• black powder (B.P.) Black power (model rocket solid fuel) motors are generally made with black powder (oxidizer and fuel). Note the black powder can't be used for reliably motors larger than "D" and "smokeless power" (gun cotton) cannot be used at all.
• caliber is the diameter of the main body tube of the rocket in question. For example, rockets are commonly 15-25 calibers in length. (This term comes from gunnery where caliber is the outside diameter of the shell.)
• CATO means that a motor blows up! This is rare for production pre-manufactured motors, but amateurs will encounter it.
• center of gravity (C.G.) is the balance point of the rocket with the intended motor loaded. You can measure the C.G. directly on a finished rocket by installing the motor and finding where it balances on an edge (like a see-saw).
• center of pressure (C.P.) is the balance point of aerodynamic forces on the rocket. You can calculate the C.P. using various rocket simulation programs. If the C.P. is not well aft of the C.G., the rocket will not be stable.
• certification The U.S. national rocketry organizations implement a system of certification, which attests that those certified have shown they can build a rocket, fly it safely and recovery it without damage at one of several levels (N.A.R. as two levels and Tripoli has three).
• ejection The charge (or sometimes mechanical system) which opens the rocket at apogee to deploy the recovery system. The ejection delay is the amount of time between motor burnout and the deployment and it timed to occur at apogee.
• engine See motor.
• fins (you knew this one) are the flat parts which stick out from the tube at the aft end of the rocket and provide stable flight. Without fins (or other special arrangements), the rocket will not fly in a straight line.
• forward is the front end of something on a rocket. Ship terminology is often used because "top" and "bottom" are confusing as orientation changes. See also aft.
• HPR High-power Rocketry (see the Categories section).
• ignition Solid-fuel motors are ignited electrically using an "ignition system." An "igniter" is inserted into the motor and when electricity is passed through it, it bursts into flame, igniting the motor. This allows the motors to be launched with everyone at a safe distance.
• impulse is the measure of thrust over time (in Newton-seconds or pound-seconds of force). The "total impulse" of a motor is the amount of energy it provides to lift the rocket and the source of the letter designation ("A, B, C" and so on).
• launcher A launcher is required to hold the rocket in a vertical position and guide it straight during the beginning of its flight (before the rocket gains enough airspeed for the fins to take effect). The most common kind of launcher is a base with a thin steel rod to which the rocket attaches with a "launch lug."
• L.E.U.P. Low Explosive Users Permits are managed by the B.A.T.F (Bureau of Alcohol, Tobacco and Firearms) and is required for purchase across state lines, storage or transportation of high-power rocket motors.
• level N "Level 1" refers to rockets which use H & I motors, "level 2" to J through L motors and "level 3" to M through O motors.
• ModRoc Model Rocketry (see the Categories section).
• motor The motive force making a rocket go. Solid fuel rockets use motors because there are no mechanical moving parts (they're not engines).
• nose The forward end of a rocket. The tapering (pointy) part of the rocket is often referred to as a "nose cone," even though the shape is rarely conical.
• October Sky is an excellent movie about a group of boys in a coal town in West Virginia who build their own rockets. October Sky is responsible for much new interest in the hobby. The book Rocket Boys by Homer Hickam, Jr. is even better than the movie (October Sky was based on Rocket Boys).
• OOP (Out Of Production) A rocket or other product no longer currently made, but remembered fondly from the past. Many discontinued Estes and Centuri rockets are highly prized.
• parachute ('chute) The most common rocket recovery system and the only one used with larger rockets. Model rocket often use flat plastic "parasheets" which are attached to the rocket with thread. Larger rockets use true parachutes because of the weight being recovered.
• recovery Rockets must be recovery safely. If a rocket comes down without a recovery system, it will fall nose down very fast and will be dangerous. Getting your rocket back in once piece is important as part of a successful flight (not to mention that it allows you to fly again). The most common recovery systems are parachutes and streamers although many others have been devised.
• shred When a rocket breaks up in flight, it's called a "shred." Rockets shred because they aren't stable, too large a motor is used for the materials, or they have not been properly constructed.
• separation When the recovery system comes out too early or too late and the rocket is still moving too fast, the recovery system takes a strong jerk. If it breaks, the two parts become detached and you have a separation.
• stability Hobby rockets almost always depend on fins and balance to guide the rocket in a straight line. A particular rocket is stable on a particular motor if it will launch and fly in a straight line. This becomes quite a complex topic (see the Rocket Design section).
• streamer A recovery system for the smallest of model rockets. Streamers are flat plastic, paper or cloth bands which are attached to the rocket and flap as the rocket comes down, slowing the descent.
• thrust is a measure of instantaneous force (in Newtons or pound of force). The "average thrust" of a motor is the average amount it pushes on the rocket during it's entire burn phase. Note that the motor generally produces different amounts of thrust as it burns and a graph of this is called a "thrust curve."

There are many other terms in use, more than we could possibly cover here, but understanding these basic will allow you to participate in rocketry conversations. See the Glossary in the RESOURCES area for many more. Another great source of information is the rec.models.rockets FAQ.