The Jargon File (version 4.4.7, 29 Dec 2003):
[SI: a quantifier meaning ? 10^-12] Smaller than nano-; used in the same
rather loose connotative way as nano- and micro-. This usage is not yet
common in the way nano- and micro- are, but should be instantly
recognizable to any hacker. See also quantifiers, micro-.
The Free On-line Dictionary of Computing (18 March 2015):
1. An SI prefix used to multiply the value of an SI
(Système International) unit by some power of ten.
2. In mathematics or programming, a
prefix operator is one that is written before its operand. In
a programming language using prefix notation, all operators
are prefix operators.