The keys defined on a X11 system running can be looked up in following file:

/usr/include/X11/keysymdef.h

To find out if the specified file above is on your running system use following command:

locate keysymdef.h

Searching for special key:

user@host~ grep -i space /usr/include/X11/keysymdef.h
#define XK_BackSpace                     0xff08  /* Back space, back char */
#define XK_KP_Space                      0xff80  /* Space */
#define XK_ISO_Partial_Space_Left        0xfe25
#define XK_ISO_Partial_Space_Right       0xfe26
#define XK_space                         0x0020  /* U+0020 SPACE */
#define XK_nobreakspace                  0x00a0  /* U+00A0 NO-BREAK SPACE */
#define XK_emspace                       0x0aa1  /* U+2003 EM SPACE */
#define XK_enspace                       0x0aa2  /* U+2002 EN SPACE */
#define XK_em3space                      0x0aa3  /* U+2004 THREE-PER-EM SPACE */
#define XK_em4space                      0x0aa4  /* U+2005 FOUR-PER-EM SPACE */
#define XK_digitspace                    0x0aa5  /* U+2007 FIGURE SPACE */
#define XK_punctspace                    0x0aa6  /* U+2008 PUNCTUATION SPACE */
#define XK_thinspace                     0x0aa7  /* U+2009 THIN SPACE */
#define XK_hairspace                     0x0aa8  /* U+200A HAIR SPACE */

Anoter useful X tool to find out a key name is called xev. This tool is part of the X.org server pacage.