The Alcock-Paczyński effect (AP effect) is an illusion affecting the determination of the depth of galaxy clusters; using galaxy redshifts with a wrong Hubble parameter makes clusters appear either flattened or elongated along the line of sight. This occurs even if there is no redshift space distortion (RSD, the effect of peculiar velocity of individual galaxies within the cluster on the apparent depth), e.g., even if each galaxy were frozen in place within the Hubble flow.
The Alcock-Paczyński test (AP test) is a use of this effect to evaluate cosmological models that produce Hubble parameters, to calibrate or disprove the model; a model's generated Hubble parameters are considered unlikely if they yield something other than a spherical galaxy cluster shape on average. Given a large enough sample of clusters, their average depth should be close to the same as their average measured width. This test has been tried, but effective accommodation of the RSDs proves to be a substantial challenge to producing convincing results.