When my son Davy (who's now eight) was about ten months old he'd cruise around holding onto the couch saying "da da da da". He'd make the same "da da da" when he wanted to play with me. As an eager parent looking for milestones, I wondered if this was his first word but decided it didn't qualify. Soon he added "dis" to his reportoire: He'd point to something and say "dis", seemingly happy if I named what he was pointing at, but sometimes wanting more. What that a word? I asked an expert on infant language. Emphatically (almost tyrannically), she said no. Then more gently she added that developmental psychologists generally didn't take such seeming-words seriously until there were at least ten of them. Then they were words.
But ten of what? A hot trend recently in certain circles is teaching babies sign language: For example, if a baby makes a fist with one hand, that means "milk", if she brings her fingertips together, that means "more". Of course we don't want to be prejudiced against sign language: Words needn't be spoken aloud. The fist sign for milk is a word.
But now suppose that we have a psychologically identical case where instead of making a fist, the baby kicks her left foot in a distinctive way when she wants milk, and the parents learn to respond to that and reinforce it. Is that left-foot-kicking a word? Presumably we don't want to say that. To count such left-foot-kicking as linguistic seems to cheapen language too much. Ordinarily we think of language as something advanced, uniquely human or nearly so (except for maybe a few signing apes and Alex the parrot). The kicking doesn't seem to qualify, any more than we say a dog has language if he gets the leash when he wants a walk. In getting the leash, he communicates non-linguistically. So where to put on the brakes? Not every human communication is a word: A red stoplight is not a word, nor is a wink or a flag or a computer icon. (I think!) ;-)
Even very young babies will tighten their fists sometimes as a sign of hunger -- but surely that's not a word for a newborn. And babies seem quite naturally to point. Is pointing a word? My newly adopted sixteen-month-old daugher Kate will raise both hands over her head to communicate that she's "all done" eating. But just as the point might be a formalized reach, the hands up might be a formalized reaching up to be lifted out of her high chair. In fact, Kate has started raising her hands over her head in frustration when she has been trapped in her car seat too long and wants to be "all done" with that.
If a twelve-month old says "cup" for cup, we call it a word. If she says "mup" for cup, we find it cute and still call it a word. It seems to follow that if she makes regularly makes a sign language signal for cup, we should call that a word; and likewise it seems that if she makes her own unique sign for cup, we should call that a word too. But now the leash and left foot kicking seem to be back in.
So what is a word?