A TeX control word is a TeX control sequence scanned from an escape followed by a maximal run of characters with TeX category code 11. The scanner then enters the space-skipping TeX input state S.[ref]
For example, after
\def\word{Q}, the input \word X yields QX; the spaces terminate the name and are skipped. The input \word X does the same. By contrast, \wordX names a single different command, so removing the separator does not preserve the token sequence.Token identity and current meaning are separate. Define
\def\call{\word}, then redefine \word to yield R. Expanding \call now yields R, because its stored command token still names word. This is useful when reading TeX macros: the spelling identifies the command, while its current definition controls expansion. New to topics? Read the docs here!