Biology and medicine •
PSI (prion), infectious protein in yeast •
Pandemic severity index, former US CDC influenza scale •
Photosystem I, protein complex involved in photosynthesis •
Pneumonia severity index •
Protein Structure Initiative, of the U.S. NIGMS
Computing •
Psi (instant messaging client), XMPP client program •
Potentially Shippable Increment, in the Scrum software development methodology •
Program-specific information, part of the MPEG transport stream protocol • Pressure stall information, a Linux kernel patch for
out of memory management
Cryptography •
Private set intersection, a cryptographic technique to compute the intersection of two encrypted sets
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 ("PS" or "pSi"), a form of silicon with introduced nanopores in its microstructure •
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==