From PanoTools.org Wiki

Angle up-and-down (above and below horizon). See also Roll and Yaw. Pitch is the angular deviation from the level (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 that the optical axis of the camera is tilted up (positive value) or down (negative value) is the pitch of the image. For example, the pitch of a zenith image (shot straight upwards) is 90°.

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