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From X-Plane Wiki

Altitude: An aircraft's altitude is its height above sea level.

CDI: Course Deviation Indicator. This refers either to the instrument used for navigating a VOR or ILS course, or more specifically to the horizontal bar found in this instrument which displays the aircraft's lateral position relative to the VOR or ILS course.

DME: Distance Measuring Equipment. An instrument used in navigation which measures distance using the delay between the sending and receiving of a radio signal. Aircraft use this to determine their distance from a fixed NAVAID.

Drag: The aerodynamic force (created by a fluid such as air flowing around an object) that slows the object's motion.

EFIS: Electronic Flight Instrument System. A flight instrument system (found in an aircraft's panel) with electronic displays rather than the steam gauges of a standard panel. In the X-Plane apps for the iPhone/iPod Touch and Palm Pre, such displays are found in jet-engined aircraft.

Glideslope: The angle at which an aircraft approaches (or needs to approach) a runway; often used when discussing navigation by instruments.

Heading: An aircraft's heading is the direction that its nose is pointing.

HSI: Horizontal Situation Indicator. This instrument is found in the jet-engined craft in X-Plane, in the left side of the panel. It serves the same function as an OBI—that is, it indicates course deviation.

IFR: Instrument Flight Rules. The procedure for flying an aircraft based solely on the craft's instrument panel. Environmental conditions requiring such flight (such as the poor visibility on a rainy day) are referred to as IFR conditions. This is contrasted with VFR conditions (those operating under visual flight rules).

ILS: Instrument Landing System. A ground-based system for guiding approaching aircraft into the runway via radio signals.

Indicated airspeed: The presumed airspeed of a craft as determined by measuring the pressure acting on a little tube attached to the craft which points into the wind. This differs from true airspeed in situations where the air has very little density (for example, at 80,000 feet in an SR-71 Blackbird or in orbit in the Space Shuttle).

Lift: The aerodynamic force (created by a fluid such as air flowing around an object) that pushes an object upward.

Localizer: A localizer is part of an instrument landing system (ILS). It serves as a guide to the centerline of the runway.

Mach speed: The speed of sound through the air. Mach's number actually describes the speed of sound through any fluid (that is, liquid or gas). In application to aeronautics, though, it is implied that the fluid is air. Note that this number is dependent on a number of factors, such as temperature, humidity, and pressure. Generally, "Mach 1" is cited as 768 miles per hour (the speed of sound at sea level in dry air at 68˚ Fahrenheit).

NAVAID: A navigation aid transmitter (in X-Plane, either a VOR or ILS) which is used as a reference when flying. These are often found near or on an airport. Pilots often fly from NAVAID to NAVAID on long flights, as a VOR is usable from about 50 miles away.

OBI: Omni-Bearing Indicator. Omni-Bearing Indicator. This instrument is found for the most part in X-Plane for iPad's general aviation aircraft. In the X-Plane applications for the iPhone/iPod Touch and Palm Pre, this is found in the piston-engined aircraft, in the middle of the second row of the panel. It consists of two moving bars (called the course deviation indicator, or CDI) which points the way to whatever VOR frequency is tuned in the navigation radio. The instrument is set using the Omni-Bearing Selector (or OBS), the knob in its lower left corner.

Pitch: Movement of the aircraft's nose up or down (see the image in the Flight Dynamics section of the manual).

Roll: Movement of the aircraft's body along the line formed by its body; in an airplane, this is easily seen as the dip or rise of the wings. See the image in the Flight Dynamics section of the manual.

Rotor: The rotating part of a helicopter that generates the craft's lift; similar in appearance to an oversized airplane propeller, though different in its operation.

RPM: Rotations per minute; a way of measuring the speed of a rotor or propeller. In a helicopter, the RPM of both the main rotor and the tail rotor are held constant.

Rudder pedals: Foot pedals in an airplane used to steer the plane down the runway and to control its yaw motion in flight (that is, the wagging of its tail left or right). Because X-Plane Mobile does not simulate rudder pedals (as they have only two axes for input rather than three), X-Plane will automatically control the rudder pedals in an attempt to stabilize the craft's yaw motion.

Speed: The change in the position of an object over time; unlike velocity, speed does not take into account the direction of the object's movement.

Thrust vector: The direction in which the engine or rotor’s thrust is going; for a helicopter sitting on a helipad with its controls at neutral, this is straight down.

Thrust vectoring: The ability of helicopters and some other aircraft (such as the Harrier or the F-22) to change the direction of the thrust from its engines/rotors.

Update: To update a piece of software is to convert it to a newer version. In the X-Plane applications for the iPhone and iPod Touch, this is done by following the instructions found here. On the iPad, this is done by following the instructions here. This should be done every couple months or so in order to take advantage of new features in the simulator.

Velocity: The combination of an object's speed and the direction of its movement; for example, an aircraft might have a vertical velocity of 500 feet per minute (meaning it moves upward at a rate of 500 feet per minute) or a vertical velocity of -500 feet per minute (meaning it moves downward at 500 feet per minute).

Vertical speed/vertical velocity: The rate at which the aircraft is gaining or losing altitude, typically given in feet per minute.

VFR: Visual Flight Rules. Flying done using a combination of the pilot's view of the outside world and the aircraft's instruments. Environmental conditions permitting such flight (such as a sunny day with 10 mile visibility) are referred to as VFR conditions.

VOR navigation: Very High Frequency Omnidirectional Range navigation; this is navigation based on radio signals sent out by a VOR beacon. Light airplanes often track these signals using an Omni-Bearing Indicator, or OBI, while more expensive craft (such as the jets modeled in X-Plane) often use a Horizontal Situation Indicator, or HSI.

Yaw: Movement of the aircraft's body left or right, most easily pictured as a wagging of the aircraft's tail. See the image in the Flight Dynamics section of the manual.