Pitch-f/x is the awesome system that MLB uses to document every single statistic about the movement of particular a pitch. However, it took quite some time to find one centralized location that provides solid documentation for all of the fields provided by the system (including ones that have been added in recent years). So for the benefit of myself and others, here it is:

• type: ball/strike/ball in play
• id: pitch ID for within that particular game
• x: horizontal location at which the ball crosses the plate in pixels
• y: vertical location at which the ball crosses the plate in pixels
• des: outcome of the pitch (18 total)
• sv_id: date and time of pitch, in YYMMDD_hhmmss
• start_speed: speed of pitch as it is 50 feet from home plate in mph
• end_speed: speed of pitch as it crosses the plate in mph
• sz_top: top of the strike zone for the particular batter in feet
• sz_bot: bottom of the strike zone for the particular batter in feet
• pfx_x: horizontal break of the pitch in inches
• pfx_z: vertical break of the pitch in inches
• px: horizontal location at which the ball crosses the plate in feet
• pz: vertical location at which the ball crosses the plate in feet
• x0: horizontal release point in feet
• y0: release point on C-2B axis (this is fixed at 50 feet)
• z0: vertical release point in feet
• vx0: horizontal velocity in feet/second
• vy0: velocity in the home plate-second base plane in feet/second (there has to be a better term for this)
• vz0: vertical velocity in feet/second
• ax: horizontal acceleration in feet/second/second
• ay: acceleration in the home plate-second base pane in feet/second/second
• az: verticaly acceleration in feet/second/second
• break_y: distance from home plate in feet at which the ball deviates most from a straight line path from release point to the point at which the ball crosses the plate
• break_angle: angle at which the ball breaks – from the catcher’s perspective, a break angle of 0 indicates no horizontal break, a positive angle indicates a ball that breaks from right to left (away from left-handed hitters) while a negative angle indicates a ball that breaks from left to right (into left-handed hitters)
• break_length: largest deviation from a straight line between the release point and the point at which the ball crosses the plate in inches
• pitch_type: type of pitch thrown
• type_confidence: confidence in the pitch_type classification by MLB on a scale from 0 to 1.5
• zone: pitch location in one of 14 zones around home plate as defined by MLB
• nasty: MLB-defined rating of quality of pitch, from 1 to 100
• spin_dir: from the catcher’s perspective, the angle (from 0 to 359.999) between the the pole around which the ball is rotating and the positive x-axis; a ball with a spin_dir of 0 has no horizontal movement and some amount of top spin while a ball with a spin_dir of 180 has no horizontal movement and back spin
• spin_rate: revolutions per minute of the ball

The two most helpful sites in compiling this information were from Alan Nathan and Fast Balls.