POSIX is an acronym.

doc/ext_jit.html

@@ -130,7 +130,7 @@ is represented by the decimal number xxyyzz.
 <h3 id="jit_os"><tt>jit.os</tt></h3>
 <p>
 Contains the target OS name:
-"Windows", "Linux", "OSX", "BSD", "Posix" or "Other".
+"Windows", "Linux", "OSX", "BSD", "POSIX" or "Other".
 </p>
 
 <h3 id="jit_arch"><tt>jit.arch</tt></h3>

src/lj_arch.h

@@ -79,7 +79,7 @@
 #elif LUAJIT_OS == LUAJIT_OS_BSD
 #define LJ_OS_NAME	"BSD"
 #elif LUAJIT_OS == LUAJIT_OS_POSIX
-#define LJ_OS_NAME	"Posix"
+#define LJ_OS_NAME	"POSIX"
 #else
 #define LJ_OS_NAME	"Other"
 #endif