Easy question...does anyone know if pp stands for anything?
The abbreviation pp stands for "plane fronted gravitational waves with parallel rays". Think of the analogy with plane waves in electromagnetism. In vacuum, a spacetime is said to be a pp wave if it posesses a covariantly constant null vector field. Covariantly constant just means that it has vanishing covariant derivative or equivalently that parallel transport of the vector field is path independent.
See here for more: http://relativity.livingreviews.org/open?pubNo=lrr-2004-9&page=articlesu28.html [Broken]
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