From PanoTools.org Wiki

Angle up-and-down (above and below horizon). See also Roll and Yaw. Pitch is the angular deviation from leveled (horizontal) shooting direction and part of the panotools Image positioning model. I.e. a rotation around the TrX axis.

If you shoot a multi row panorama you will have to tilt the camera up or down. The angle the optical axis of the camera is tilted up (positive value) or down (negative value) is the pitch of the image. The pitch of the zenith image (shot straight upwards) is 90° f.e.

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