transport stream protocol Pressure stall information, a Linux kernel patch for out of memory management Economics PSI-20, Lisbon, Portugal stock market index Private sector involvement, in sovereign debt crisis resolution Engineering "Yaw" angle, one of the Euler angles, denoted ψ in aerospace engineering Present serviceability index, a pavement performance indicator introduced by AASHTO Geography Psi Islands, in the Melchior Islands, Antarctica Mathematics Chebyshev function Dedekind psi function Digamma function Polygamma functions Stream function, in two-dimensional flows Polar tangential angle of a curve Probability of ultimate ruin, in ruin theory Supergolden ratio Reciprocal Fibonacci constant Population Stability Index (Kullback–Leibler divergence#Symmetrised divergence) Physics and chemistry Pound per square inch, unit of pressure PSI (computational chemistry), software J/psi meson, subatomic particle Porous silicon Water potential, denoted Ψ, in physical chemistry, the potential energy of a water solution relative to pure water Wave function, denoted ψ, in quantum mechanics Psychology Psychology or psychologist, denoted Ψ, used in the logo of various psychological associations Psi Chi, the National Honor Society in psychology Psi-theory, theory about the human mind, a cognitive architecture Psychiatric Solutions, Inc., corporation that owns psychiatric hospitals Other uses Parasocial interaction, non-reciprocal interactions, as between a fanbase and a performer Permanent staff instructor, in the British Army Personalized System of Instruction Pollutant Standards Index, air pollution measure Pre-shipment inspection, method of quality control Presentence investigation report, investigation into the history of person convicted of a crime See also Psy (disambiguation) Ps1 (disambiguation)