19.2.1 Synonymy and Antonymy

When the meaning of two senses of two different words (lemmas) are identical or nearly identical we say the two senses are synonyms. Synonyms include such pairs as:

couch/sofa vomit/throw up filbert/hazelnut car/automobile

A more formal definition of synonymy (between words rather than senses) is that

two words are synonymous if they are substitutable one for the other in any sentence without changing the truth conditions of the sentence. We often say in this case that the two words have the same propositional meaning.

While substitutions between some pairs of words like car/automobile or water/ $ H_{2}O $ are truth-preserving, the words are still not identical in meaning. Indeed, probably no two words are absolutely identical in meaning, and if we define synonymy as identical meanings and connotations in all contexts, there are probably no absolute synonyms. Many other facets of meaning that distinguish these words are important besides propositional meaning. For example $ H_{2}O $ is used in scientific contexts, and would be inappropriate in a hiking guide; this difference in genre is part of the meaning of the word. In practice the word synonym is therefore commonly used to describe a relationship of approximate or rough synonymy.

Instead of talking about two words being synonyms, in this chapter we will define synonymy (and other relations like hyponymy and meronymy) as a relation between senses rather than between words. We can see the usefulness of this by considering the words big and large. These may seem to be synonyms in the following ATIS sentences, in the sense that we could swap big and large in either sentence and retain the same meaning:

(19.15) How big is that plane?

(19.16) Would I be flying on a large or small plane?

But note the following WSJ sentence where we cannot substitute large for big:

(19.17) Miss Nelson, for instance, became a kind of big sister to Mrs. Van Tassel's son, Benjamin.

(19.18) Miss Nelson, for instance, became a kind of large sister to Mrs. Van Tassel's son, Benjamin.

That is because the word big has a sense that means being older, or grown up, while large lacks this sense. Thus it will be convenient to say that some senses of big and large are (nearly) synonymous while other ones are not.

Synonyms are words with identical or similar meanings. Antonyms, by contrast, are words with opposite meaning such as the following:

long/short big/little fast/slow cold/hot dark/light rise/fall up/down in/out

It is difficult to give a formal definition of antonymy. Two senses can be antonyms if they define a binary opposition, or are at opposite ends of some scale. This is the case for long/short, fast/slow, or big/little, which are at opposite ends of the length or size scale. Another groups of antonyms is reversives, which describe some sort of change or movement in opposite directions, such as rise/fall or up/down.

From one perspective, antonyms have very different meanings, since they are opposite. From another perspective, they have very similar meanings, since they share almost all aspects of their meaning except their position on a scale, or their direction. Thus automatically distinguishing synonyms from antonyms can be difficult.