The
C (and, by
inheritance of accummulated
features,
C++,
Perl,
Python,
Java, and who knows what other
UN*Xoid
tools)
prefix for a
hexadecimal numeric literal. In other words,
0x123 == 291 in C.
It's not a computer science thing at all, just a convention in a part of the computing world which has grown very large in the past few years. Most assemblers used $, BBC BASIC used &, and even an H suffix have been used.