A word is one of the basic syntactical units in Tcl. A word is a string of characters, and a command is a list of words.
AMG: The difference between speaking of a command as a list of words versus a list of arguments is whether or not the name of the command is intended to be included in the list, with words including the name. Also note that the name of the command may comprise multiple words, as in a command ensemble.
A command prefix is an example of a list of words because it must, by necessity, include part or all of the name of the command, in addition to including zero or more of the arguments to the command.