Character you have to type to start wildcard expansion in the command-line, as specified with 'wildmode'. More info here: cmdline-completion. The character is not recognized when used inside a macro. See 'wildcharm' for that. Some keys will not work, such as CTRL-C, <CR> and Enter. <Esc> can be used, but hitting it twice in a row will still exit command-line as a failsafe measure. Although 'wc' is a number option, you can set it to a special key:

:set wc=<Tab>

NOTE: This option is set to the Vi default value when 'compatible' is set and to the Vim default value when 'compatible' is reset.

(Vim default: <Tab>, Vi default: CTRL-E)