pS

Ps, written with an initial capital P and a lowercase s in many contexts, is a shorthand used for several concepts across different fields. In the Greek alphabet, psi (Ψ, ψ) is the 23rd letter and is widely used as a symbol in science and mathematics, notably to denote the wavefunction in quantum mechanics.

In measurement, ps stands for picosecond, a unit of time equal to 10^-12 seconds, common in ultrafast physics and electronics. In printing and publishing, PostScript, abbreviated PS, is a page description language developed by Adobe for representing complex page layouts and graphics. In writing, P.S. (postscript) is a short remark added after the main text in a letter or email. In computing, ps is the Unix command that reports currently running processes. In popular software contexts, PS is a common shorthand for Adobe Photoshop, an image editing program; the native file format is PSD (Photoshop Document).