Name the enumeration PitchType, PitchKind, PitchMagnitude, PitchQuality, PitchShape, PitchSpeed, PitchStrength, or anything that works best.
Another consideration is whether class design can be improved. Instead of having the PitchType property inside the Pitch class, you can also create a class hierarchy:
public abstract class Pitch {}
public class Fastball : Pitch {}
public class Sinker : Pitch {}
public class Curveball : Pitch {}