What’s a word?  It seems almost silly to ask such a simple question, but if you think about it, the question doesn’t have an obvious answer. A famous linguist named Ferdinand de Saussure said that a word is like a coin because it has two sides to it that can never be separated. One side of this metaphorical coin is the form of a word: the sounds (or letters) that combine to make the spoken or written word. The other side of the coin is the meaning of the word: the image or concept we have in our mind when we use the word. So a word is something that links a given form with a given meaning.

Linguists have also noticed that words behave in a way that other elements of mental grammar don’t because words are free. What does it mean for a word to be free? One observation that leads us to say that words are free is that they can appear in isolation, on their own. In ordinary conversation, we don’t often utter just a single word, but there are plenty of contexts in which a single word is indeed an entire utterance. Here are some examples:

What are you doing?  Cooking.

What are you cooking?  Soup.

How does it taste?  Delicious.

Can I have some?  No.

Each of those single words is perfectly grammatical standing in isolation as the answer to a question.

Another reason we say that words are free is that they’re moveable: they can occupy a whole variety of different positions in a sentence.  Look at these examples:

Penny is making soup.

Soup is delicious.

I love to eat soup when it’s cold outside.

The word soup can appear as the last word in a sentence, as the first word, or in the middle of a sentence.  It’s free to be moved around.

The other important observation we can make about words is that they’re inseparable: We can’t break them up by putting other pieces inside them. For example, in the sentence,

Penny cooked some carrots.

The word carrot has a bit of information added to the end of it to show that there’s more than one carrot. But that bit of information can’t go just anywhere:  it can’t interrupt the word carrot:

*Penny cooked some car-s-rot.

This might seem like a trivial observation – of course, you can’t break words up into bits! – but if we look at a word that’s a little more complex than carrots we see that it’s an important insight.  What about:

Penny bought two vegetable peelers.

That’s fine, but it’s totally impossible to say:

*Penny bought two vegetables peeler.

even though she probably uses the peeler to peel multiple vegetables.  It’s not that a plural -s can’t go on the end of the word vegetable; it’s that the word vegetable peeler is a single word (even though we spell it with a space between the two parts of it).  And because it’s a single word, it’s inseparable, so we can’t add anything else into the middle of it.

So we’ve seen that a word is a free form that has a meaning.  But you’ve probably already noticed that there are other forms that have meaning and some of them seem to be smaller than whole words. A morpheme is the smallest form that has meaning. Some morphemes are free: they can appear in isolation. (This means that some words are also morphemes.)  But some morphemes can only ever appear when they’re attached to something else; these are called bound morphemes.

Let’s go back to that simple sentence,

Penny cooked some carrots.

It’s quite straightforward to say that this sentence has four words in it. We can make the observations we just discussed above to check for isolation, moveability, and inseparability to provide evidence that each of Penny, cooked, some, and carrots is a word. But there are more than four units of meaning in the sentence.

Penny cook-ed some carrot-s.

The word cooked is made up of the word cook plus another small form that tells us that the cooking happened in the past.  And the word carrots is made up of carrot plus a bit that tells us that there’s more than one carrot.

That little bit that’s spelled –ed (and pronounced a few different ways depending on the environment) has a consistent meaning in English: past tense.  We can easily think of several other examples where that form has that meaning, like walked, baked, cleaned, kicked, kissed. This –ed unit appears consistently in this form and consistently has this meaning, but it never appears in isolation: it’s always attached at the end of a word. It’s a bound morpheme. For example, if someone tells you, “I need you to walk the dog,” it’s not grammatical to answer “-ed” to indicate that you already walked the dog.

Likewise, the bit that’s spelled –s  or –es (and pronounced a few different ways) has a consistent meaning in many different words, like carrots, bananas, books, skates, cars, dishes, and many others.  Like –ed, it is not free: it can’t appear in isolation.  It’s a bound morpheme too.

If a word is made up of just one morpheme, like banana, swim, hungry, then we say that it’s morphologically simple, or monomorphemic.

But many words have more than one morpheme in them: they’re morphologically complex or polymorphemic. In English, polymorphemic words are usually made up of a root plus one or more affixes. The root morpheme is the single morpheme that determines the core meaning of the word. In most cases in English, the root is a morpheme that could be free. The affixes are bound morphemes. English has affixes that attach to the end of a root; these are called suffixes, like in books, teaching, happier, hopeful, singer. And English also has affixes that attach to the beginning of a word, called prefixes, like in unzip, reheat, disagree, impossible.

Some languages have bound morphemes that go into the middle of a word; these are called infixes. Here are some examples from Tagalog (a language with about 24 million speakers, most of them in the Philippines).

It might seem like the existence of infixes is a problem for our claim above that words are inseparable. But languages that allow infixation do so in a systematic way — the infix can’t be dropped just anywhere in the word. In Tagalog, the position of the infix depends on the organization of the syllables in the word.