The Greek letter psi (Ψ or ψ) is used for angles in biochemistry.
In Foldit, psi and the closely related phi generally refer to the two critical angles of the protein backbone. Phi and psi are graphed in the Rama Map. For a given segment of the protein, psi is the angle between the alpha carbon and the peptide bond of the next segment, and forms the vertical y-axis of the Rama map. See Ramachandran Map for more.
Upper- and lower-case psi generally have a "trident" shape that's easy to recognize. In some fonts, the prongs of the trident curl, while in others, they're relatively straight. It's sometimes hard to distinguish lower-case psi from lower-case phi, but the prongs of the trident in psi are always open, while phi has a loop that's almost completely closed.