It is a type definition for length and size values used by socket related parameters, return values and structure members. Such definitions are used to allow platform independant source files. The header file where it is defined (
sys/socket.h) may use different definitions according to the target platform (e.g. for 16, 32, and 64 bit CPUs).
See also
http://pubs.opengroup.org/onlinepubs/009696699/basedefs/sys/socket.h.html[
^].