Synonymy holds between senses, not between words. “Keen” and “sharp” are synonyms in the sense of drawing fine distinctions — a keen mind, a sharp mind — and not synonyms in the sense of harsh, where sharp pairs with tart and keen does not appear at all. A thesaurus that pools all senses of a word into one list produces sets whose members cannot substitute for each other in your sentence.
What a synonym actually is
In the lexical database behind this dictionary, meanings are the primary objects. A meaning is a set of words that can express it, and that set is the synonym list. “Sharp” does not have synonyms; each of its 15 senses does.
The consequence is immediate and easy to miss. Two words are synonyms only relative to a sense, so the true statement is never “keen means sharp”. It is “keen and sharp share this sense and not that one”.
That is why every synonym list on this site is printed inside a sense rather than at the top of the entry. It is more work to render and it is the only arrangement that is true.
The example, in full
Sharp, sense 3, is “having or demonstrating ability to recognize or draw fine distinctions”. Its recorded synonyms are acute, discriminating, incisive, keen and knifelike. So keen is a synonym of sharp — here.
Sharp, sense 5, is “harsh”. Its recorded synonyms are sharp-worded and tart. Keen is absent. A sharp remark is not a keen remark, and no amount of the two words being “synonyms” makes that substitution work.
Keen has senses sharp does not reach at all — sense 3 of keen is “very good”, with synonyms bang-up, bully, corking, cracking and dandy, a set nobody would arrive at from the word sharp. And keen has a noun sense, a funeral lament, which shares nothing with any of it.
What pooling does to a thesaurus
Flatten a polysemous entry and you get a list where most pairs are not substitutable. “Fine” has 10 senses here. Sense 1 is “being satisfactory”, with synonyms all right, o.k., okay and hunky-dory. Other senses cover thinness, fineness of texture, and purity of a metal.
Pooled, that entry offers hunky-dory as a synonym of fine. Which is true — for one sense out of ten. Apply it to “a fine thread” and you get nonsense, and the thesaurus that handed it to you gave no indication which of the ten it came from.
This is the single most common way a thesaurus makes writing worse. The replacement is not a word the writer misunderstood; it is a word the tool presented without the context that made it correct.
Three things a synonym still does not carry
Even inside the right sense, substitutability is not guaranteed. Sense-grouping fixes the largest error, not every error.
- Register. 610 entries carry an informality label and 202 are marked slang. Within one sense, one member may be neutral and another unusable in a report.
- Region. 918 entries carry a British usage label, 126 an American one. A synonym that is ordinary in one place is conspicuous in the other.
- Collocation — which words a word habitually stands next to. “Strong tea” and “powerful tea” describe the same thing and only one is English. No lexical database records this, including ours, and the only reliable source for it is reading.
Antonyms work differently, and more strictly
6,621 entries here carry a directly recorded antonym. That is a much smaller number than the synonym coverage, and the reason is that antonymy is recorded between specific word forms rather than between meanings in general.
Hot and cold are recorded as antonyms; hot and chilly are not, even though chilly and cold share a sense. The opposite relation attaches to the pair, not to the concept, and it does not travel across a synonym set.
That makes antonyms unusually reliable when they exist. If two words are listed as opposites here, a lexicographer recorded that pair, and antonym pairs are the most efficient vocabulary to learn together because each one anchors the other.
How to actually use the entries
Find the sense you mean first, then read only its synonyms. On a word page the senses are grouped by part of speech and numbered, and each carries its own synonyms, antonyms, broader terms and narrower terms in its own block.
If your candidate replacement looks promising, open its page and check which of its senses your sense corresponds to. A word borrowed from sense 3 of one entry and dropped into sense 1 of another is exactly the failure this whole arrangement exists to prevent.
And when the entry lists no synonym for a sense — which is common — that is information, not a gap. The more precisely a sense is drawn, the fewer other words land on it.
Questions
- Are synonyms always interchangeable?
- No. Synonymy holds between senses, so two words can be synonyms in one meaning and unrelated in another. Keen and sharp are synonyms for “drawing fine distinctions” and not for “harsh”.
- Why does this dictionary list synonyms under each meaning instead of once at the top?
- Because a single pooled list is wrong for every sense but one. An entry with ten meanings has ten synonym sets, and merging them produces a list whose members cannot replace each other in your sentence.
- How do I choose between two synonyms in the same sense?
- Check register, region and collocation in that order. Two words in one sense can differ in formality, in which country they are ordinary, and in which words they habitually appear beside. The last of those is not recorded in any lexical database and only reading supplies it.
- Why do some senses have no synonyms at all?
- Because the sense is drawn precisely. A meaning that only one word expresses is the normal case for specific vocabulary, and an empty synonym list is an accurate statement rather than missing data.