The concept of “word” would seem fairly central to linguistics. One of the definitions of “syntax” given by the Oxford English Dictionary is:
“The ways in which a particular word … can be arranged with other words …”.
And “morphology” is defined:
“the structure, form, or variation in form … of a word or words …”.
Semanticists talk about “word meaning”, phonologists about “word stress” and so on. All this is all very well – but what is a “word”? This question, it turns out, is akin to most other questions in linguistics in not being answered as easily as we might like. A big part of the problem arises because of conflicts between different criteria for wordness. Take, for example, the element ‘m in I’m. From a purely grammatical point of view, ignoring the sound (and writing) side of things, ‘m acts like a word – shown most clearly by the fact that it can always be substituted with am with no real change in meaning: I’m playing means the same as I am playing, and so forth. (am shows much more wordy behaviour.) But ‘m isn’t like words in other respects: it doesn’t contain a vowel, and it can’t occur on its own. Thus while am needn’t immediately adjacently follow I, ‘m must:
- OK: Am I playing?; I probably am playing
- Not OK: ‘M I playing, I probably’m playing
Other items like ‘m in English are things like the ‘ll of I’ll be playing, the n’t of isn’t, hasn’t etc., the ‘s in the king of France’s head and so on. These can be called “clitics”. One definition of a clitic is that it is a grammatical word but not a phonological word. Grammatically, ‘m behaves like am, ‘ll behaves like will and n’t behaves like not, so they can be said to be grammatical words*. But they can’t appear on their own: they must form a single phonological unit with another item, being pronounced (and written, not completely incidentally) as if they were part of it. Neither can they bear stress, like more typical words:
- OK: You must NOT go (emphasis on not)
- Not OK: You mustN’T go (emphasis on n’t)
On these sound-based criteria, then, clitics don’t seem to be words. There are a lot of complications here and I’m oversimplifying some issues slightly, but it’s hopefully clear that the issue of a word is isn’t terribly clear-cut. To make matters worse, some items seem able to be both full words and clitics – e.g. the usually doesn’t bear any stress and is pronounced quite weakly, like a clitic, but sometimes it is stressed, shown most clearly in something like I didn’t say A book, I said THE book. And there’s dispute whether some items in some languages are clitics or just inflections.
To conclude, then, the idea of a word is somewhat complicated. Some things behave like words in some ways but not others; some words can sometimes be substituted for things that are not words, or at least not on all criteria. If there is a moral, it is that even the most basic concepts (in linguistics, and presumably elsewhere) cannot necessarily be taken for granted.
* – Possessive ‘s is a bit more complicated. There’s no full word that can be substituted for it, unless you change the order of things to get the head of the King of France, and even then the two aren’t totally equivalent. But unlike items like plural –s (e.g. in kings) it doesn’t attach to words but whole phrases: we say the kings of France but not the king’s of France head. As it isn’t a word by all criteria and also isn’t an affix like plural -s, it gets lumped in the clitic category.
Dixon, R.M.W., & Aikhenvald, Alexandra. 2003. Word: A Cross-linguistic Typology. Cambridge: Cambridge University Press. See particularly the introduction.